feet on the ground

Strong evidence has been found that the first hominins - the fossils found from the period immediately after the split between the lineages - walked on two feet, but there is a dispute as to whether they completely gave up the trees and moved to live in the African grasslands, or continued, for a time, to do both - running on the plains and climb the trees

"Descent from the trees" has long become one of the most common clichés of evolution - the moment that symbolizes our transformation from apes to humans. But of course, in reality evolution was much more gradual, and the lineage that eventually led to us, humans, included animals that were not monkeys or even apes, but were also very different from modern humans. These species, which have been extinct for a long time, are called by the group name (along with the only species that did not become extinct, us): "hominins". According to fossils, as well as genetic studies, the lineage that led to us split from the lineage that led to chimpanzees, our closest carnivores, 5 to 7 million years ago. But what happened since then?

Paleontologists (fossil researchers) can determine the walking path of the animals they study by the bones of the foot, ankle, pelvis and more. Strong evidence has been found that the first hominins - the fossils found from the period immediately after the split between the lineages - walked on two feet, but there is a dispute as to whether they completely gave up the trees and moved to live in the African grasslands, or continued, for a time, to do both - running on the plains and climb the trees

Anthropologist Jeremy DeSilva from the University of Michigan decided to look at the issue from a different angle. The question he posed is - what was actually needed for the ancient hominins, in order to live in the treetops. All apes that exist today live in forests, and trees are their natural environment - therefore, even though the common ancestor of us and chimpanzees was definitely not a chimpanzee, most researchers believe that he resembled them in his lifestyle, and was an expert like them in climbing. In order to obtain information about the possible climbing ways of that mysterious ancestor, de Silva examined the climbing form of chimpanzees in a reserve in western Uganda. His main finding concerns the angle between the foot and the calf - during the climb up the trunk, the chimpanzees bend their foot towards the leg, up to an angle of 45 degrees from the resting position - compared to an angle of only 15 to 20 degrees in humans. More precisely, a person's foot can bend much more than that - but the ankle in question will have to spend several weeks in a cast afterwards. It may seem like a rather insignificant detail, but this bending is very important to the climbing ability of the chimpanzees, and also of the other great apes - gorillas, orangutans and gibbons in zoos have been seen climbing in a similar way. Of course, the apes' foot and ankle bones are different from ours, and adapted to their special climbing ability.

And what about the ancestors of humans? De Silva studied 14 shin bones (tibia) and 15 ankle bones (talus) from fossils 4.1 million years old (nearly after the split between the lineages) and up to 1.5 million years old. All the bones looked very similar to the bones of... modern humans. According to the bones, the first hominins simply weren't able to perform the bending so important for climbing. Although, it is possible to climb without bending the foot - it seems that monkeys (as opposed to apes) climb this way, but since the researchers assume that the common ancestor of humans and chimpanzees climbed the trees like the modern chimpanzee and the other apes, the change that took place in the bones of our ancestors seems to indicate A change in the environment they lived in, from the trees to the ground. According to these findings, the descent from the trees was made relatively quickly - the first hominins committed themselves to life on the ground and gave up the trees almost completely.

De Silva himself admits that his research is not conclusive - if the basic assumption is wrong, and the common ancestor of chimpanzees and humans climbed in a different way, without bending the leg, it is possible that the first hominins kept this form of climbing, and were expert climbers despite what their leg bones claim . And as another anthropologist, William Jungers, commented to Science magazine - there are also modern humans who are able to climb trees very easily, without a noticeable change in their ability to bend their feet. But the fact that all other great apes are endowed with the same trait, which only we lack, gives strong support to the sharp transition theory - so maybe the old cliché is true, and there really was a single evolutionary moment, when we "came down from the trees".

For information on the subject on the Science website