Questions
Questions 1-5 are based off the PLOS One paper entitled "Early Human Speciation, Brain Expansion and Dispersal Influenced by African Climate Pulses" by Shultz and Maslin (2013).
The pulsed climate variability hypothesis is one of four explanations currently being debated as to what caused the drastic increase in hominin diversity, migration, and increase in brain size 1.8-1.9 Ma. It describes areas of East Africa that fluctuated rapidly between extreme wet-dry conditions. More specifically, it is signified by the East African Rift System (EARS) spanning a distance of 4500 km and the rapid flooding and drought that led to changes in vegetation from trees and shrubs to tropical grasses. Other hypotheses include the savannah hypothesis, which would lead to increased aridity and increased savannah grassland; variability selection hypothesis, in which environmental instability led to further hominin development; and finally the turnover pulse hypothesis, where minor environmental changes lead to differentiation in speciation and adaptation. While the author does recognize the pulse hypothesis as the closest of the three, the evidence of the pulsed climate variability far outweighs the findings on the turnover pulse hypothesis. Such findings show that lake basins (especially in EARS regions) were extremely prevalent, there were shifts in vegetation, and an increase in the temperatures of the Pacific and Indian Oceans would have caused more moisture in the tropics. While the pulsed climate variability hypothesis provides solid research findings, the authors concluded that it should be viewed as a starting point for the explanation of hominin development during this time period. While the climate and environment would have played a vital role in the species development, it may be too simplistic at this point to encompass all the environmental forces during this time.
2. What happened in the subfamily Homininae 1.9-1.8 Mybp? Why is this so important in our evolutionary history?
During the period 1.9-1.8 Ma, hominin diversity and development exploded. The period is characterized by increases in species diversity, migration out of East African and into Eurasia, and increases in brain size. This time in our evolutionary history was extremely important because it has allowed us to evolve to where we are today. The changes in the diversity of hominins led to the first Homo erectus, thus allowing us to become bipedal walkers. The migration out of East Africa and into Eurasia helped increase diversity by subjecting populations to different environmental factors that would lead to further speciation and adaptation. And finally, the increase in our brain size and development allowed hominins time, something they had never had before. With an increase in brain size, they were able to focus on "technologically advanced" ways of hunting, such as throwing stones at animals. This would have allowed them to be more efficient hunters, consequently allowing them to obtain the nutrition needed to further their development.
3. If changes as seemingly minor as lake presence and dust deposition can have these dramatic implications, what might you predict will happen as a result of current anthropogenic climate change?
If minor changes like lake presence and dust deposition were able to create dramatic events such as hominin speciation and dispersal events, then it can be postulated that the current anthropogenic climate change can have dramatic implications as well. Some areas of the world may receive increased rainfall which could lead to increased productivity, while still more may receive less rainfall, resulting in decreased productivity and shifts in habitat availability. Likewise, with the rate at which the anthropogenic climate change is occurring, some rural areas could see dispersal events as the result of flooding or dried-up river basins. Over time, it is possible that these events could create dramatic changes that lead to speciation, just like that of the African climate pulses.
4. Apply Darwin's postulates to the adaptation of hominins to climate change.
Charles Darwin's postulates are: 1) Individuals within a species are variable; 2) Some of these variations are passed on to the offspring; 3) In every generation, more offspring are produced than can survive; 4) The survival and reproduction of individuals is not random, natural selection operates on populations; and (although not included in the text, Darwin also included) 5) Through the accumulation of different adaptations, different groups would become species. The first postulate says that there needs to be variation within a species which can be seen in the differences in brain size in hominins. According to the second postulate, these variations need to be able to be passed on to offspring, which we have seen in an 80% increase in brain size during this period. Thirdly, there needs to be more offspring produced in a generation than are able to survive. This postulate is slightly more difficult to track due to the lack of evidence in the area, but can be inferred that populations were migrating to Eurasia due to a low quality of life in which many of their offspring would not have been healthy enough to survive birth. Darwin's fourth postulate says that natural selection occurs on populations which can be seen from various angles. The climate was posing environmental stress on hominins during this time period and the EARS lake system required frequent fluctuation. As a result, geographically isolated populations evolved independently of one another. And finally, the fifth postulate states that through the accumulation of different adaptations, new species would result. This can be seen during this time period, one in which the species were more diverse than they ever have been.
5. What is an adaptive radiation? What evolutionary processes do the authors believe are responsible for the described adaptive radiation in hominins?
According to the Berkeley Evolution website, adaptive radiation is described as an event that causes species to rapidly diversify. This diversification can be caused by a wide range of factors and can result in the formation of new lineages. The authors believe that adaptive radiation could have been caused by one or more of three possible hypotheses. The first idea proposed is that the hominins were located in areas that were spatially constrained due to geographic isolation that would have resulted in population isolation and vicariance. Some specific examples of this first hypothesis are noted in the paper being the East African Rift System (a series of lakes that experienced vast extremes from flooding to drought) as well as mountain and volcanic ranges (such as Barrier Volcano, Namaruno volcano, e.g.). The second hypothesis proposed suggests that the hominins were occupying novel niches that were highly productive in aiding development. The East African Rift System (EARS) lakes in these areas were able to provide large deep fresh water lakes that would have allowed these hominins a specific niche to reside with ample water and food supplies. Finally, the last hypothesis mentioned in the paper was that the instability and drying of the EARS lakes could have led to periods of environmental stress resulting in strong selective pressures against the populations.
Shultz, Susanne, and Mark Maslin. "Early Human Speciation, Brain Expansion and Dispersal Influenced by African Climate Pulses. "PLOS ONE:. Faculty of Life Sciences, 16 Oct. 2013. Web. 03 Apr. 2014
"Triggering Adaptive Radiation." Evolution 101: The Big Issues. Berkeley, n.d. Web. 03 Apr. 2014.
