Blog Five: Bipedalism and Arm-to-Leg Length Ratios

The hominin traits I will be discussing are bipedalism and arm-to-leg length ratio.

As we learned this week, there are several species that came about from the divergence of chimps (or chimp-like apes) and humans. Each species had a different set of hominid traits. One single species did not all of the sudden possess all the traits of the Homo Erectus. This was due to the different selection pressures each species faced. Some needed to adapt due to predators so they changed their form of locomotion, and some to the new kinds of food they had to eat so their molars, incisors and canines grew and shrank to varying sizes.

The two forms, robust and gracile, separated and formed the species we studied about this week.

Though the gracile forms had more in common with the characteristics and traits we hold today, those qualities were still varying in different forms of the A. Afarensis.

While species like he Orrorin tugenesis, Ardipithecus and possibly the Sahelanthropas were all bipedal, they either had small teeth, long arms, or even brains the size of chimpanzees – meaning that while they may have walked like an early human, that was the only similarity and the rest of them would have been more primitive, or related more closely to ape-like primates than humans.

Bipedalism came about in forested environments and were the result of adaptation to avoid predators.

The evolution of bipedalism came about slowly however, in small increments. In other words, several of the hominins with bipedal locomotion also still retained their long arms and legs which meant that they did not solely travel on two legs but also through the trees.

Varying forms of hominins had more use of their bipedal movement or more use of their tails and long arms and legs.

For example, the Orrorin tugenesis had long ape-like arms but also operated with bipedal legs. Evidence showed that the species lived in dry, evergreen environments – the kind of place where bipedalism first emerged, making Orrorin possibly one of the first bipedal species in our ancestry.

As time went on, arm and leg lengths of hominins gradually grew smaller to adapt to bipedal life. The A. Garhi species was one of the first species to have similar-sized body parts that we have today. The teeth, arms and legs, and head size were all premonitions to the hominid body of the future. However, the brain size remained small and chimp-like in operation.

This is why paleoanthropology is so interesting and helpful in reconstructing the past.

By discovering the skeletons of different species, we can come to the conclusion that our evolution was of a mosaic nature, and not all at once, which better helps us understand how we came to be.

Through determining the age of the fossils discovered, we can more easily pinpoint which trait came about first, and which divergence of a species adapted the latest. Building on the different species that each held traits and characteristics that we have today can help anthropologists piece together the timeline of our evolution.


One thought on “Blog Five: Bipedalism and Arm-to-Leg Length Ratios

  1. I really like how you explained the evolutionary change to bipedal-ism being slow and gradual. I never thought about how the slow gradual change in climate would cause hominids to need both adaptions of walking on all fours and bipedal-ism. The forests slowly changed into savannas so to did the trait adaptions on hominids for use in both environments. I think the gradual change in traits allowed for development of a hominid that could adapt to their niche in the environment, where each generation slowly changed to deal efficiently with their surroundings. Survival was important, and the best traits were passed onto future generations that are present in modern humans. Each generation builds on the last, if scientists can study these divergences, we can pinpoint at what times these traits changed in the fossil periods. And, a better understanding of how modern human evolution came about.

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