06/04/2020- Why did monkeys never continue evolving?

    A common question for primatologists to answer is: if humans evolved from monkeys, then why are there still monkeys?

    What people forget about evolution is not that a species is becoming more "advanced" or it is becoming "smarter"; when a species develops something, it is because it helps that species continue to exist and reproduce in that specific environment. Just because humans evolved from chimps, that does not make chimps any less successful than humans.

    Modern humans evolved with their needs at the time. Chimps of that time did not need to change because they were successfully reproducing in the environment.

    But what was this environment like that forced ape to evolve traits that developed into a series of different species called hominins, which are animals more closely related to humans than to apes? 

    The simple answer is that we do not know, yet. There are multiple hypotheses, the most popular being the Savannah Hypothesis, the Turnover Pulse Hypothesis, the Forest Hypothesis, the Variability Selection Hypothesis, and the Pulsed Variability Selection Hypothesis.

    a. The Savannah Hypothesis 

    This was coined by no other than Charles Darwin himself, who said the expansion of the savannah, which decreased forest cover, force apes to evolve traits to move out of the trees and onto land. This would make sense as to why bipedalism evolved, which is much more energetically efficient for a terrestrial lifestyle. However, Early bipedal hominins were found in wetter, forested environments, which obviously does not support Darwin's thoughts. However, when looking at other animals, there is evidence to support cooler and drier conditions (so, more grasslands). 

    b. The Turnover Pulse Hypothesis

    Elisabeth Vbra explained that there were a lot of extreme and fast environmental changes occurring, and apes had to evolve more generalized diets to survive harsh times in which all primates were feasting on the same foods. This argument states that hominins evolved bipedalism to be able to eat in the trees and on the ground, and carry food back to others with the two freed up hands. Evidence at the time showed that ungulates, which are hooved animals that have a generalized diet, survived the environmental turbulence better than specialized species. So, in theory, hominins evolving bipedalism, and subsequentially a generalized diet, helped them survive a wider range of environmental conditions.

    c. The Forest Hypothesis

    R.J. Rayner believed forested environments were key to bipedalism, and that the last universal common ancestor used an arboreal form of bipedalism, where the animals walked across the branches on two feet and used their arms and hands to balance themselves. Fossils from the time show pollen evidence that they lived in a wooded habitat.

    d. The Variability Selection Hypothesis

    Similar to the Turnover Pulse Hypothesis, Richard Potts notes that high climatic variability could have caused behavioral and morphological changes to help hominins survive a wider range of environments. 

    e. The Pulsed Variability Selection Hypothesis

    Finally, the East African Rift System and deep lakes are key to understanding hominin evolution, as the lakes are affected by monsoonal rains and solar precessional cycles. These heavy rains and cycles influenced aridity and humidity, and formed gigantic lakes that were a barrier that allowed the diversity of hominins.

    We don't know why humans evolved the way they did, but we do know that other primates didn't because they were surviving well in their environment. Now, if monkeys were removed from forests and placed in deserts, then we may see either an extinction or a newly evolved species, but as it is right now, monkeys are doing and have been doing fine just as they are.