The Cenozoic mammal record and onto the Late Pleistocene and the migration of hominins across the globe
The Evolution of Maximum Body Size of Terrestrial Mammals
Smith et. al 2010
According to this paper, it has been shown that the rate, trajectory, and upper limit of body size have remained relatively constant in pattern across different continents, orders, and guilds of species. It is suggested that the filling of niches, although a significant hypothesis alone, is strengthened alongside factors such as temperature and land area available that could have made these ultimate causes for the evolution of maximum body size in terrestrial mammals.
The maximum body size for terrestrial animals when they first evolved was small in comparison to the maximum size after the K-Pg mass extinction. So we must ask, what about the K-Pg contributed to such large terrestrial animals to evolve after such a large blow in species? A significant recovery period must have had to take place, but what factors led to mammals that were up to thousands of pounds rather than the 1-33 pound mammals that existed when mammals first came into existence? A thorough analysis of the maximum size of terrestrial animals across different continents, taxonomic groups, lineages and feeding groups was done using primary sources that showed that there was a pattern for body size across continents, lineages and trophic groups. On each continent studied, body size drastically increased during the beginning of the Cenozoic following the K-Pg, and continued to increase thereafter.
Another analysis of body mass over time between herbivores versus carnivores showed that there is a pattern of increasing size of both groups, but herbivores have a higher maximum than carnivores. This may indicate that there is an indirect relationship where the maximum carnivore body size follows the size distribution of mammals. More importantly, the study of global fluctuations in global temperatures, atmospheric oxygen levels and terrestrial land area were analyzed. Each of these had the same increasing and then saturation trends, but it seems that global temperature and terrestrial land area were most important. Increasing terrestrial land area indicated that there were lower sea levels, therefore more terrestrial land and more room for animals to populate and reduce their chance for extinction. This relates to temperature because the low sea levels most likely were due to water that was stored in ice caps. This must mean that the largest terrestrial animals were evolved during periods in which Earth was cool. Along with these trends, it was found that larger mammals were found on continents that were larger which matched up with trends in the land area over time vs body size over time.
Questions:
1) What kind of ecological niches would have been filled for larger mammals to evolve?
2) What are some of the physiological and ecological constraints placed on carnivorous mammals that make their maximum body size smaller than herbivorous mammals?
3)Why do you think the K-Pg mass extinction provided the ecological opportunity for mammals to become larger? (In your own words)
Evolution of the Earliest Horses Driven by Climate Change in the Paleocene-Eocene Thermal Maximum
Secord et. al 2012
This paper discusses the importances of mammalian physiology and ecology. Body size in particular is able to tell us a lot about the physiological/ecological components of animals on its own. Similarly to the article by Smith et. al, trends in body size over time for a number of genera were studied. The time periods that were compared were the Paleocene and the Eocene, which were the first two epochs (respectively) to follow the K-Pg extinction into the Cenozoic. They analyzed body size of some of the oldest forms of horses under a variety of environmental pressures such as temperature, aridity, and partial pressures of CO2. These trends prompted the researchers to suggest that there drier conditions followed by wet conditions (precipitation) and then followed again with the dry conditions. Along with that, the body size of extant horses showed a decreasing trend. One suggestion was that since there were higher CO2 levels during times of decreasing horse body size, there must have been more plants but with reduced nitrogen and protein content within them. This led to the hypothesis that the herbivores were eating less nutritious and hard-to-digest plants that allowed for this stunt of growth trend. This hypothesis isn’t the likeliest, but could have been a contributor. Global warming along with high CO2 concentrations seems to be more of a pressing factor on body size.
Questions:
1) What do you think warming/high CO2 does physiologically or ecologically to cause a decrease in body size?
2) Do these environmental factors described in the article alone give you good reason to believe that they were in fact causes for the body size decrease? If you don’t think there is enough evidence, what would you suggest as another alternative hypothesis for body size decrease in the Paleocene-Eocene?
Review: Out of Africa, the peopling of continents and islands: tracing uniparental gene trees across the map
Stephen Oppenheimer 2012
This paper discusses the various methodologies and difficulties of determining the migration patterns of anatomically modern humans (AMH). Early genetic relationships between humans were studied using the population based approach; although this approach proved to support African ancestry for AMH, there were difficulties with resolution and dating. The lineage based approach is based on analysis of uniparental non-recombining DNA, such as in mitochondrial DNA (mtDNA) or on the Y chromosome, and analyze highly specific mutations on single locus phylogenies to trace migrations and the exit route of AMH from Africa. The construction of a worldwide beta-globin locus phylogeny based on population genetics resolved Africans as the deepest single ancestral branch from which all non-African groups arise, pointing toward a single exit event of AMH out of Africa.The movement is hypothesized to be along coastlines, which allowed for rapid migration and dispersal, which gave rise to three mtDNA haplotypes (M, R, N).
The paper suggests that although genetic phylogeography is useful for topographic questions on migration and route, archaeology, paleoanthropology and earth sciences, when combined, would be more precise in increasing confidence for inferring time depth of the individual branches. Phylogenetic inferences are combined with founder analysis to assemble gene trees made by mutations on non-recombining DNA branch points, where migrations can then be inferred using distribution of these gene branches and the diversity along these branches then support the time depth of these individual branches.
The multiregional hypothesis states that regional populations each descended from largely separated migrations of Homo erectus and the morphological resemblance between populations is due to modern gene flow. Under the multiregional hypothesis,non-African populations should possess more recent lineages and a mixture of ancient lineages, signaling multiple exits of AMH from Africa. The recent replacement model states that AMH originated from African stock around 200ka ago, and replaced other human groups in and outside of Africa within the last 100ka. Under the recent replacement model, older lineages would be present in Africa, while young lineages would be found exclusively outside of Africa.
The L3 branch is not unique to Sub Saharan Africa (SSA), as it encompasses all non-Africans. The M and N branches , the only two L3 branches which originate outside of Africa are the only two branches found throughout the rest of the world. This indicates that the rest of the world was colonized by descendents of a single ex-African founding mtDNA haplotype, which can be traced back 71,600 years. These two branches suggest a single successful exit of AMH from Africa.
Y chromosome evidence and mtDNA evidence show similar phylogeny with a single mutation (M168) in the African CDEF branch that gives rise to all non-Africans. The similarities between these two groups supports a single exit most likely following a southern route. However some studies have suggested by looking at Y chromosome branches that a DE branch as the D branch is uniquely East Asian but also part of the original exiting clade (CDEF), combined with the Y chromosome branch E is one of the most common clade in Africa, makes the argument that E left Africa much later raising doubt with a single exit from Africa. This causes some differences with the mtDNA as mtDNA shows that L3 as a single exit support. However these differences can be explained by L3 splitting into M and N and Y chromosome splitting into C, D and F, could have happened prior to the exit of Africa. This split would have left E behind before the exit. This means that one DE mutated and later became the Asia D claude while the other DE remained in Africa and later became E claude. Relic groups near the Indian ocean also support the single exit model as these groups are descendants of mtDNA L3 and Y chromosomes C and F. There was however black flow of the branches back into Africa as evident by the mtDNA M branch. This branch in Ethiopia is much newer than branches found outside of Africa suggesting that this branch is a descendant from a non-African claude rather that the older African claude. This backflow is also supported by Y chromosome data showing F and E, but E is most likely East African origin but F makes up 10-40% of backflow in North and East Africa.
The route that was most likely taken was one of two routes, the Northeast route to Egypt through the Sahara and an East route that ultimately leads to South Asia. of these two routes the east route towards Asia was the most likely as this route as population of Europe would have to had taken place after population of South Asia and this would then mean European mtDNA lineages would be younger than South Asian lineages. This is in Fact the case as the mtDNA lineages that split from L3 outside of Africa (M and N), show that the European N linage is indeed younger than the South Asian lineages. Another Factor that makes the route to South Asia more likely is the climate condition present at the time of both paths. The Northeast path would have to have crossed through the arid Saharan-Arabian zone that was present 90-48 ka, but this obstacle would have also been present in the East towards South Asia path. The South Asia path however it was proposed that fresh water aquifers would have been present on the coastline on the South Asia path during times of low sea levels.
Migration to Europe would have to have taken place once the population could move over the arid zone. A partial amelioration of climate happened around 48 ka that would cause increased humidity that would allow for populations to cross this arid zone and reach Europe. This migration however is thought to have been unsuccessful as the Y chromosome phylogeography and dating is not similar to this population's prevalence in the gene pool. This exit though does give us evidence that another southern Eemian AMH exit was correct, despite a lack in genetic evidence. Some evidence that suggest a successful migration include fossil records and cultural evidence connecting a AMH with a Eemian exit out of Asia.
The colonization of Asia would have been prior to the YTE, but was interrupted by the eruption and thus caused a slow recovery in India. This is supported by a completer change in mtDNA haplogroups between Eastern India and Indo-China. This is shown with a higher prevalence of unique M groups in East India with little N groups. Also these M groups where younger others along the Indian ocean trail. In groups further west had higher N and R group prevalence, which is similar to the pre-Toba exit. There is though no fossil evidence of pre-Toba AMH dispersal to SE Asia.
Non Carbon dating has suggested that Australia could have been colonized as early as 50-60 ka. Sea- level curves do suggest that that an opportunity of colonize Australia would have been greatest 65ka, when sea levels were low enough to cross the Sahul shelf. CS-mtDNA sugest that Australia and New Guinea was colonized at a similar time due to the presence of P mtDNA similar to Asia claude found in both populations. The differences in these populations is most likely due to their isolation.
Questions:
- Would humans have been able to diversify as rapidly had they not migrated via coastlines, and would there have been a reason for them to if they hunted/used a more abundant food source?
- Had humans taken longer to populate the globe, would there be as large of a diversity within the human species as there is today? (i.e. Would all races have had time to develop?)
- Do you think the multiregional hypothesis or recent replacement model is a more feasible explanation for widespread diversity of AMH?
- After reading this article do you believe that the fossil record is the only reliable way to track past migrations, or do you think that other methods should be considered (even though they are less accurate in some way) in order to determine past migrations. Why?
- Do you think multiple exits models or single exit model is more likely based on the evidence of this paper. What are the limitations or flaws in each?
- After reading this paper and getting a better understanding of ancestry tracking, how accurate do you believe the modern ancestry testing to be? Do you think it's fairly accurate and mostly scientific or do you feel it is more on the statistical side and should be taken with caution? Explain.
To answer your KP-G question. It is because that extinction event opened up all the niches for large bodied organisms and mammals were lucky enough to fill those niches. I'm not sure what you mean by developed races. Were you talking about civilized populations versus hunter gatherer? The eastern side of the world still would have held an advantage over the western in terms of accelerating human development. If the timing had been extended for global population it is possible that we could have been at higher risk to being wiped out to some unforeseen environmental cause. If something like that did not occur, it still would only have been a matter of time before humans conquered the globe. I think with modern advances in science our ancestry tracking is only going to get better. We are able to determine that we share 98% of our genome with chimpanzees and they are just a relative. That being said if we can see in our genome genes that are shared with chimps from a common ancestor well before humans had evolved every human is going to share genes with each other.
ReplyDeleteSmith:
ReplyDeleteThe smaller average size of carnivores as compared to herbivores is highly due to their position in the food chain and how the transfer of energy in this system works. Organisms at higher levels obtain less overall usable energy from their food source because only a percentage of energy is actually stored by the body after consumption. Therefore, at each higher lever, a lower amount of energy will be obtained to be used for metabolic processes. Consequently, carnivores must be smaller in order to supply their physiological processes with the small proportion of energy gained from their herbivorous prey.
As we discussed in class, a large contributing factor to the increase in mammalian body mass following the K-Pg was likely due to the drop in global temperatures. The early Oligocene was a time of major cooling and glaciation. Not surprisingly, this is also a time of mammal turnover and the evolution of larger forms. Because of their lower surface area to volume ratio, larger mammals do not have as much of a problem with heat exchange in colder environments. They are therefore able to retain more energy that would have otherwise been dissipated in order to maintain an optimal body temperature.
Secord:
Similarly to above, this correlation between warming and smaller body size has to do with variations in heat exchange. Smaller organisms have a larger surface area to volume ratio which causes them to easily lose heat to their surroundings. Therefore, in warming environments, it is physiologically favorable to be smaller.
Smith et. al
ReplyDelete1. For larger mammals to evolve, the food sources
at 'smaller' areas would have had to be occupied. Caves and burrows fit smaller animals as well. I hate to be basic here, but animals like giraffes have evolved larger bodies and longer necks to take advantage of a food source that smaller animals simply cannot reach. That being said, carnivores also cannot get as large since the transfer of energy is only about ten percent (hence why enormous mammals like whales eat such small things like krill).
2. The transfer of energy from food source to consumer is about ten percent from what I understand. So an herbivore eating a plant gains about ten percent of the energy. A carnivore eating an herbivore gets ten percent of the energy of the herbivore, or one percent of the original energy. So a carnivore needs to be smaller to be able to sustain itself with lower amoutns of energy.
3. So, the giants of the time were the dinosaurs. They were large towering creatures that nobody small would dare to mess with. They filled a niche- a large animal niche. Despite not being mammals, the space was occupied. They were dominant. When they all died out, there was no dominant towering animal group. It gave the mammals the space they needed to fill the niche of 'large towering animal that has little to no predators.'
Secord et. al
1. Well, for once thing, with high CO2 there's less O2 in the air and larger animals have a harder time with gas exchange than smaller animals, hence why insects and worms can respirate through their skin. Anyway, with warming, heat exchange changes- larger animals cannot survive in higher temperatures, whereas smaller animals can. They are better suited for cooler temperatures, so warming hurts them.
2. Honestly I think that temperatures definitely are a huge factor when it comes to body size, and I'm content with the environmental factors mentioned here.
Stephen Oppenheimer
1. Humans probably would not have diversified as rapidly since there would not have been any adaptive radiation if they had all stayed in the same place. There would have been no reason to migrate if there was an abundant food source, unless populations got so big that they needed to expand.
2. I think that humans would still be as diverse, even if it had taken longer to populate the globe, since the environments are still so different. However, this sort of depends on just HOW long it had taken. They still needed time to adapt.
3. The recent replacement seems to be more feasible to me.
4. The fossil record is notoriously scarce, since not everything fossilizzes. When exploring scientific research, there should always be more than one way to test things. I do not believe it is the ONLY reliable way, though it would be incredibly useful. As for other methods of tracking...I suppose it would be very difficult to track this in other ways.
5. I want to believe in a multiple exit model, but the single exit model seems more plausible based on just how difficult it would have been to traverse the continents. The limitation for that is just how widespread diversity of AMH. However, the flaw in the multiple exit model is the fact that traversing the continents would have been difficult. To be honest, I have no idea about this.
6. As with everything, it will probably continue to improve over the years to an extent. I don't think anything will ever be perfect, but I think it's fairly accurate and scientific. However, with how mainstream it has been recently, it should only get better.
Smith et al. Paper:
ReplyDelete1. The ecological niches that would have to be filled in order to support larger mammals would most likely have to deal with flora in the area. There would have to be enough digestible vegetation for the herbivores to eat and survive on as they evolved to have larger body sizes. The size of the carnivores that preyed on the large herbivore mammals would follow.
2. The constraints placed on the carnivores would be based on their predatory skills in relation to their increasing body size. At some point, they will become large enough to not be able to move fast enough or act stealthily enough to catch their prey and therefore their body size will be forced to remain smaller than the mammal herbivores they prey on.
3. I think the KPG provided the opportunity for the mammals to become larger because the extinction of the large terrestrial dinosaurs left a lot of ecological niches open and ready for colonization.
Smith:
ReplyDeleteJust like with all mass extinctions, there are large populations that used to cover an entire environmental niche and they don’t anymore. With the K-Pg, most dinosaurs were apart of entire niches and they had gone extinct. This allowed mammals to take over those niches, feed on more abundant food sources, and not have some of those predators to stop them. This increased the size of mammals and they started to become the dominant population groups in certain niches.
Secord:
With the warming of temperatures, organisms with high surface area to volume ratios would end up surviving better. This would include all the smaller organisms and they would be able to have a greater heat exchange compared to larger bodied organisms. This would account for body size decreases as the temperatures started to rise.
Stephen:
I believe that humans would not have been able to diversity as rapidly if they had not migrated. This would all be based on available niche spaces. Without migration, humans would occupy the same niche spaces in large clusters and adaptive radiation would not be able to happen. Migration allowed humans to live in many different environments and allowed for diversity to occur on a quicker scale.
It is abundantly clear that anatomically modern humans are the descendants of African ancestors. The use of genetic haplotypes, complementing archeological evidence, has honed a phylogeographic model that points to a single migration out of Africa, with the possibility of a recolonization by some of the earliest migrants. The mtDNA and Y chromosomal DNA analyses, presented by Oppenheimer and others, convincingly indicate an initial southern route for these migrants, bringing them along the coastline through India into the Indonesian archipelago and then Australia. Subsequent proliferation of these groups brought them all the land masses on the globe.
ReplyDeleteBecause Homo erectus was found in Europe, the Middle East, Africa, and Asia, it is reasonable to assume that those populations experienced levels of parallel evolution with Homo sapiens in Africa. Similarities were probably more pronounced than differences, among the distant cousins. The inclusion of autosomal H. s. neanderthalensis and H. s. denisova DNA in AMH leads to one spectacular image of human evolution. There was multi-regional evolution of H. erectus into humans, but the African species replaced them (with some mixing). No Homo floresiensis mixing occurred, and it is likely there were other humans that lost out to AMH. Bad genes or bad luck?
Smith: Carnivorous mammals are carnivorous, so they gain their energy by eating other animals. This means that the maximum body size it can be, relies on how much intake they can get. Metabolically, it doesn’t make sense to make a carnivore as big as an elephant, because it would constantly have to be hunting and eating to maintain homeostasis. Herbivores, on the other hand, gain their energy by eating plants. Unlike animals, plants can’t run away. This allows herbivores to become larger than carnivores.
ReplyDeleteSecord: I think warming causes body size to decrease because the smaller an animal is, the easier it can dissipate heat. The surface area to body volume ratio increases as you get smaller, so the amount of heat a hamster can release relative to an elephant is much larger. I’m not too sure that warming would cause ecological changes that would decrease body size, but anytime you change something globally, a lot of things change which could cause a domino effect.
Smith: Carnivores would have had to be fast, and energy efficient to catch other animals. Because of these two constraints, having a smaller body size would have helped, but it would also have been helpful in terms of hiding and the element of surprise when hunting.
ReplyDeleteWhen the larger animals went extinct many of the other animals either went extinct or became fewer in numbers, niches opened up for mammals to become larger. There were more opportunities for mammals to grow larger, as their competitors and predators were gone. Also, grasslands expanded across the continents, allowing mammals to radiate into these larger spaces and to grow faster, stronger and larger.
Secord: The body size of horses decreased as the available oxygen in the atmosphere was decreased and CO2 increased. So these mammals developed smaller bodies to better utilize the oxygen available. Also, they needed to develop teeth that could graze the silica laden grasses that were becoming more prevalent in their environments before they could utilize the nutrition available in this food.
Oppenheimer: Much of migration seems to be driven by food sources and if humans had developed the hunting of a larger variety of prey in one area, they may not have diversified as they had and had taken longer to populate the globe. The fossil record is certainly a very strong indicator of how migrations happened, but other approaches provide other information and alternatives, filling in the gaps left by the fossil record, which is by no means complete.
I think that open niche space was one of the most important factors in the post KPg radiation of animals. If there is open niche space life will find a way to fill it. Carnivores do not have access to primary resources like herbivores so their size is limited by the nutrients they can take up from their food. Temperature is an important factor in body size because of surface to volume ratios but it's not the only factor that controls body size. The fossil record is not the only way to track human migrations with the development of phylogenetics there are advanced methods to track these things.
ReplyDeleteThis comment has been removed by the author.
ReplyDeleteThe ecological niches that were filled may have been mammals that were grazers. There massive body size may have filled these niche spaces to prevent them from being attacked by carnivorous mammals that were a quarter, or less of their size. In order for carnivorous mammals to have any type of agility they would have to compensate in size, because most carnivorous mammals are territorial they may have had limitations to their food sources. According to the Smith article I would have to say a shift to cooler temperatures had something to do with the potential growth of mammals, as well as, a way to defend themselves from predators.
ReplyDeleteDuring the K-Pg almost all large organisms seem to have been killed off, or have gone extinct, so to say the actual cause in the decrease of body size is based high CO2 I would not be able to agree with this, because according to Wilf 2006 the end Cretaceous Extinction created high Nox, and higher than normal acid rain concentrations that increased tannin levels, and nutritional value, which can also reduce growth in organisms that may consume them. So if nutritional values were low during this time I would have to say high CO2 may have not been a factor in the decrease in their growth.
1) Larger niches would have probably been taken over from the large mammals. If we think about the dinosaurs and how large some of them were, it would make sense that other large mammals would be able to survive in a larger niche that was occupied by other large animals.
ReplyDelete2) I don’t think humans would have been able to diversify as rapidly if they didn’t use the coastlines. I think being in a spot that gives easy access to the ocean and to the land gave them more opportunity for survival. I believe they would have more food resources from both the ocean and the inland sources because there would have been a variety. It wouldn’t have been as dangerous on the coastline, but being inland probably would have made them more vulnerable to many threats that they didn’t run into on the coastline.
3) I think the ancestry tracking today is fairly accurate and includes scientific information because we have to use our own DNA to get results. I think it can be off a little, but for the most part I believe it is an accurate way of testing our ancestry.
As we've discussed in class, the extinction of the dinosaurs opened up niches for mammals to fill and helped contribute to the evolution of large-bodied mammals. The Smith et al. paper further analyzes mammal body size and links it with both land size and temperature. It makes sense that a larger land size and colder temperature combined with recently opened niches would allow for the rapid adaptive radiation seen in mammals after the K-Pg.
ReplyDeleteFor the Secord et al. paper, I believe higher CO2/rising global temperatures could lead to a decrease in body size because there would be more competition for limited resources. If an area only has limited resources but has many different species vying for those same resources, then having/evolving a smaller body size would give you a higher chance of ensuring the survival of your species. In addition, having a smaller body size is usually correlated with having a shorter gestation period during pregnancy, and developing into a mature adult at a quicker rate than larger mammals. These would both help you to continue your lineage despite competition for resources.
If humans had taken longer to populate the globe, I still think that we would have eventually diversified and developed all of the many races today because of sexual selection and the different environments that humans settled in. Humans occupied many different ecosystems, including the coastlines and humid areas, sub-humid, semi-arid, arid, etc. All of these climates are different, and I believe this is part of what led different cultures to value certain resources above others. At the same time, different groups have different criteria for sexual selection, so different traits become dominant in different societies. I know that these aren't the only factors for diversifying race, but I believe they play a role in this process.
ReplyDeleteSmith et. Al:
In order for carnivorous animals to be successful in pursuit of food sources, they need to be able to be a good hunters. Having the ability as good hunters to be swift and fast is a major factor in how they can obtain their food sources. If the animal is too large this may slow them down, therefore, decreasing their odds of being consistently good hunters to keep up with their metabolic needs. As we talked about in class, the ability for legs to become longer would have helped animals become faster to obtain their prey. I think one of the main reasons that mammals were able to become larger after the K-Pg was the development of widespread grasslands. This gave an opportunity for mammals to find more food sources and thus lead to more niches being filled.
Secord et. Al:
With rising levels of CO2 and decreasing levels of O2 this favored a decreasing body size in mammals. This is because gas exchange is more difficult with increasingly warming temperatures for larger animals. As discussed in class, higher surface area to volume ratios would favor smaller animals. When it comes down to it warmer temperatures play one of the most influential factors on maximum body size favoring smaller animals.
Oppenheimer 2012:
Had humans not used the coastline to migrate, or even split to where some used the coastline and some not use the coastline, human would still have diversified just not as rapidly. The challenges become greater when the coastline was not utilized, in regards to food sources and knowing exactly where you are going. The human curiosity to know where alternative routes lead to as opposed to the coastline, may have sparked a lot of interest to some humans. Had humans taken longer to populate the globe, the initial rate of diversity would be lower, but eventually it would speed up due to occupied areas becoming more habitable. In regards to tracking past migrations, the fossil record is not the only accurate way to track past migrations. With many older organisms that are extinct today the fossil record may be the most accurate way to study past migration. With regards to humans, it may be better to use alternative methods such as studying mitochondrial DNA, or studying specific chromosomes. Because of how recently in time humans have originated, this makes chromosomes or mitochondrial DNA a great candidate to study. I personally think after reading this article, ancestry tracking would be accurate, and I would consider getting my ancestry lineage tracked. Based on the scientific methods in which lineages were measured, I think it would give a person a fairly accurate ancestral background.
1) I think that, naturally, all of the ecological niches which had been previously taken by other species would be available to mammals, including the large carnivore niche. Since the largest species in the Cretaceous were predominantly dinosaurs that were wiped out during the K-PG extinction, I think we can confidently say that the elimination of the dinosaurs allowed mammals to take over resources and niches left behind.
ReplyDelete2) Over time, CO2 and warming temperatures may cause a decrease in body size because larger animals have a smaller surface area to volume ratio and have more trouble surviving in warmer environments compared to smaller animals, which have high surface area to volume ratios. I think the environmental factors described in this paper provide a good explanation for body size decrease, but I've also read a few papers for a project recently that comprise metadata analyses in favor of modern climate change leading to decreased body size over the past century, which gives me more confidence in believing that warming and high CO2 can lead to decreased body size.
Smith Paper
ReplyDeleteCarnivorous animals will almost never be bigger than herbivores, because of several factors which include environmental (carbon sources) and economical (cost of largeness). Carnivores will tend to be smaller than herbivores, because they are only get energy from a secondary carbon source (herbivores) whereas herbivores get there energy from a primary source (grasses/ foliage) so herbivores get a lot more bang for their buck and have to work much less to find/catch their energy then the carnivores do. Carnivores also economically can’t get too big because if they are ever a lot bigger than their pray they will need to catch many prey items per outing in order to meet their choleric needs, whereas smaller predators that only need to catch one or two prey items have the advantage in that specific situation.
Oppenheimer Paper
I think ancestry tracking can be both mostly scientific and should be taken with caution. I see how ancestry may mean a lot to some people as it helps them discover who they are and how they can learn about their families past and whatnot, however we do also need to realize that just because a lot of our ancestors have come from one place doesn’t mean we are any less human or that there geographical regions change who we are compared to others as we are all one species no matter how some people see it or act. Ancestry is often used to justify arguments between people or groups of people concerning who has what right to land or their superiority over others, and while it is a scientific process of figuring out where your DNA resides in history, it does not in any way help these arguments as the overarching thing we need to remember is that we are all one species no matter what we look like so really ancestor means nothing besides giving you a last name if that’s the custom your ancestors followed.
In regards to the first paper by Smith et. al., after the K-Pg mass extinction there was a large niche left open for the mammals to adaptively radiate. This was because the largest animals on Earth before the K-Pg were in the dinosaur families whereas the mammals were all relatively small. After the dinosaurs started to go extinct at a rapidly high rate, there was more resources becoming available, larger uninhabited areas, and less predators. The mammals were then able to rapidly diversify and overall become much larger in body size.
ReplyDeleteIn the second paper it talks more about how the climatic temperatures would have affected mammal diversification and body size. Since mammals are sensitive to heat loss and heat conservation, the rising CO2 and climate warming would have affected how large an animal can become.
The third paper brought up a lot of interesting research. I think that the single exit hypothesis makes more sense for the research that was brought up in this paper. The paper talks about how people most likely traveled along coast lines when first venturing out of Africa. This could have been for many reasons. If fishing was an option then then traveling along the coasts makes sense so that there is always an available food source. Another thought is that they traveled along the coastal areas because there are less predators than hiking around in dense forests or open grasslands. The last though that I had about why they would travel along the coast is that it is easier to tell where they have already been and where they are heading. When traveling around in dense forests it is hard to know if you are going around in circles or heading in the direction you wish to be traveling in.
Smith et. al 2010
ReplyDeleteIt seems likely that the smaller, scavenging niches would have largely been filled even after the KPG extinction, so mammals were forced to occupy new roles to them by evolving larger body sizes and developing new feeding patterns. The primary driver for the evolution of giant mammals was diversification to fill ecological niches. Prior the KPG mass extinction, mammals were confined to small body sizes and therefore limited niches. Carnivores are necessarily smaller than the herbivores they prey on due to physiological and ecological constraints. Some of these constraints could be due to the high energy demands associated with being a carnivore. The KPG mass extinction culled many animals, leaving ecological space to be filled in by the survivors of the mass extinction, mammals. Having the ability to expand is rare in a normal environment, but the mass extinction provided that opportunity to mammals.
Secord et. al 2012
Secord found that animals body size does indeed respond to temperature fluctuations, with hotter environments producing smaller body sizes and colder climates producing larger body sizes. This pattern could be due to the need for heat conservation or loss, or it could be related to the animal’s productivity in a particular temperature or environment. I believe the temperature changes discussed in the article do give enough support for a significant change in body size in animals. Over millions of years, it makes sense that mammals would evolve a body size well suited to their environment’s temperature.
Oppenheimer 2012
I believe humans were able to expand faster due to their migration along coastlines, as opposed to mainland routes, simply because their chances for survival and success would be higher near water sources. Seeking alternate food sources may have been a driving factor for their expansion, but additional factors such as a rapidly expanding population could have also driven the migration of humans out of Africa. No matter how slowly or quickly humans had migrated across the continents, I believe races would have developed in respective regions, simply due to limited access to ancestral populations.
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ReplyDeleteQuestions:
ReplyDelete1) What do you think warming/high CO2 does physiologically or ecologically to cause a decrease in body size?
I believe high CO2 concentrations most likely is correlated to body size decrease not only by the effects on the body but on the environment. Under greater CO2 concentrations, like the paper states, the plants had reduced nitrogen and protein content. This phenomena limits the extent of the physiological growth for mammals as they cannot get the additional nutrients needed.
2) Do these environmental factors described in the article alone give you good reason to believe that they were in fact causes for the body size decrease? If you don’t think there is enough evidence, what would you suggest as another alternative hypothesis for body size decrease in the Paleocene-Eocene?
I think that the hypothesis that herbivores were eating less nutritious food as a result of higher CO2 levels is a plausible additional cause of decreasing body size. I do not believe that this is the sole reason and there were very likely other more immediate effects of rising CO2 levels. I think the hypotheses linking the changing landscape to decreasing body sizes are much more explanatory.
1. Would humans have been able to diversify as rapidly had they not migrated via coastlines, and would there have been a reason for them to if they hunted/used a more abundant food source?
I think the coastline movement is essential to migration diversification. The coastline has less ecological restrictions and allowed them to follow food sources more successfully.
2. Had humans taken longer to populate the globe, would there be as large of a diversity within the human species as there is today? (i.e. Would all races have had time to develop?)
I think this is an interesting question but I think that the diversity would still exist.
3. After reading this article do you believe that the fossil record is the only reliable way to track past migrations, or do you think that other methods should be considered (even though they are less accurate in some way) in order to determine past migrations. Why?
I think that looking at DNA after this paper is a lot more crucial and essential to understanding migration of AMH. With how limited the fossil record can be, the DNA can provide the information to fill the gaps.
1) What kind of ecological niches would have been filled for larger mammals to evolve?
The increased land area would have caused multiple ecological niches for larger animals to evolve. The lower sea levels allowed for animal’s geographical ranges and perhaps diets to expand leading to a larger body size. I would expect much of these ecological niches to resemble grasslands or any type of open space as plant life would have been restrained to smaller forms as sea levels decreased.
2) What are some of the physiological and ecological constraints placed on carnivorous mammals that make their maximum body size smaller than herbivorous mammals?
Carnivores face ecological restraints determined by their biological need. Carnivores must have a smaller body size than that of herbivores as their growth is restrained by the availability of prey. If carnivores were larger than herbivores, they would have to attain much more prey to fulfill their biological needs which would be a very tiresome and unsustainable cycle.
3)Why do you think the K-Pg mass extinction provided the ecological opportunity for mammals to become larger?
I believe the k–Pg mass extinction provided the ecological opportunity for mammals to become larger through changing the terrestrial landscapes of environments which allowed for more dynamic interactions between predator prey interactions and the expansion of ecological niches.
Smith et al. 2010
ReplyDelete1.) In order for larger animals to have evolved to the sizes they were they would have had to obtain resources that were inaccessible to small and medium sized consumers. In other words, all other niches would have had to have been filled to drive evolution for some creatures to fill vacant niches.
2.) One could argue that herbivores have a far greater abundance of food that is easier to access. They are more likely to eat consistently than let's say, a lion, who may go a few weeks without a successful hunt. In this case, it simply boils down to access of food sources. Physiologically, because carnivores have to chase down food, they have greater metabolic needs to be met. When they do not replenish their energy stores, their body must compensate with whatever they have in their body. Herbivores probably do not use as much energy to obtain food, but may need more of it. Carnivores have evolved in a way that allows them to be fast and agile enough to catch prey. As compensation for speed, they must sacrifice body size.
3.) The K-Pg opened up an abundance of niches. The K-Pg caused the extinction of many species who left niches vacant. This provided the opportunity for mammals to take advantage of the situation.
Secord et al. 2012
1.) I would say that warming temperatures and high CO2 affect the respiration of vertebrates. Oxygen rises as temperatures warm, which could be a result of an influx of CO2. One could infer that if oxygen rises, due to warming temperatures, then there should be less oxygen towards the ground. If there is less oxygen available for animals to use during respiration, they must adjust physiologically to live in the changed conditions. This would suggest that animals will become smaller when less oxygen is available, to properly function.
2.) I would agree with the evidence provided, in the text, that such environmental factors, such as elevated Co2 concentrations and warming temperatures, were stronger drivers of evolution.
Oppenheimer et al. 2012
1.) I do not believe human would have diversified as rapidly. The reason being is that when they began their migration, they were able to take advantage of far more resources. They were not confined to what they had, previously, in Africa. If they had used more abundant resources in Africa, then chances are they wouldn't have needed to diversify until they depleted their resources.
2.) Chances are, no. Had they not populated the Earth in the time they did, the diversity of the human species, would be much less. Reason being, it takes time to diversify. The longer time extends, the more time a species has to reach new lands and conform to the new environment.
4.) The fossil record itself is probably inefficient to track past migration patterns. The record itself is often broken and lacks key components. However, it is difficult to contemplate other ways that would be more efficient. Perhaps using DNA analysis would suffice.
5.) I do not think that the migration of humans was done by a single unit. I like to think that there were multiple parties that found different routes out of Africa. It would have been difficult to do so, however, a single route taken out of the continent does not seem like it would give rise to the diversity seen in human populations. This could be an explanation for how the single exit theory is flawed. In the case of the multiple exit theory, the landscape and conditions would have proven difficult for several human populations to bypass.
In order for large niches to be filled by mammals the large bodied dinos would have needed to go extinct or even have the numbers of individuals brought to an all-time low. Large bodies mammals filled the niches in the early Cenozoic that the large dinosaurs did in the Mesozoic only because of their extinction. Now let’s be clear according to this paper and evidence it was the large bodied herbivorous mammals that were mainly filling these niches, such as elephant like mammals and so on; leaving the carnivores smaller in size and this may have been (in my opinion) due to the availability in food for the herbivores being greater than that for the carnivorous mammals. Thus, meaning that that with the warmer temperatures leaching over from the K-Pg mass extinction the environments would have been hotter and had a high CO2 concentration in the atmosphere. Body sized decreased as the amount of available oxygen was decreased due to the increase in CO2, thus mammals were smaller in order to use the available oxygen more efficiently.
ReplyDeleteMigration seemed to be driven by the availability in food source. If early humans had developed the skill or taste for hunting for a large variety of mammals in a centralized area, I propose that mammals would not have diversified as they did and they would have taken longer to distribute around the globe. I think that the fossil record is a good indicator as to how and possibly when migratory pattern took place as well as understanding what their food source of choice was.
Simply, large predator and herbivore (higher trophic levels) would have been freed for mammals to fill.
ReplyDeleteCarnivorous mammals must be quick moving and lightweight to hunt strenuously in a scuffle or for long periods of time stalking or chasing. Being too large would mean difficulty of fast movement and less endurance. Herbivores, however, need to appear challenging as prey and require large body size as resistance to carnivorous predators.
This was because the reigning clade – reptiles – were removed from their trophic throne and mammals faced significantly less threat. They were able to become the new apex predators and herbivores, having the ecological space to grow larger and become dominant in the same way dinosaurs were.
High temperatures mean reduced metabolism and difficulty of long-term activity. Smaller organisms would have smaller surface area-to-volume ratio which would keep them cooler in a hot environment than if they had a large ratio.
These environmental factors contribute adequately, in my opinion. Temperature would significantly shape chemical processes and metabolism development, meaning the structure of an ecosystem would be different in cold vs. hot environments. Geographic range is an obvious limiting factor, as a species can only grow to what their environment can support.
Humans would likely have not diversified as rapidly without this massive increase in geographic range because this revealed more shelter and nutrients to humans as they moved down coasts. Humans could have used resources in one area more sustainably, but they did not have such knowledge at the time. There likely would not be as much human diversity if this migration took longer than it did, because the evolutionary time scale would have been pushed back. These types of migrations can be examined in the fossil record in terms of extinctions or vegetative loss/impact, but archaeological and geological evidence are highly valuable in understanding what changes in a landscape over time, especially human landscapes because we change our environments so significantly. I think that modern ancestry tracking is messy and inaccurate because it is based upon name lineages, which don’t really describe the entire history of an individual.