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An Excerpt from Ancient Bones

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The Primal Ancestor: Still an Ape or an Early Hominin?

In an article published in 2017, I, along with my colleagues Nikolai Spassov, David Begun, and Jochen Fuss, put forward the hypothesis that Graecopithecus was no longer a great ape but the earliest potential hominin. This hypothesis was based on the fact that this species had typical hominin teeth. Apart from upright gait, this is one of the few characteristic features of the human lineage (Hominini) on which experts more or less agree.

The idea was well received by the general public but, as expected, got a mixed reaction in the world of academia. The places where El Graeco was found (Greece and Bulgaria) did not fit the accepted idea that the crucial advances in human evolution had taken place in Africa and nowhere else.

It is also generally thought that the geographic distribution of a species cannot confirm a group’s lineage and therefore can be of no help in answering the question of whether El Graeco was an early hominin or not. The only useful evidence when tracing a group’s origins are physical features or what lies in its genes. It is impossible, however, to extract genetic material from fossils that are millions of years old. And because we have not found any footprints from Graecopithecus to give us more information about the way it walked, the only way we can answer the question of whether El Graeco really was the oldest known hominin is by using physical evidence from fossils. It is clear that we need more finds if we are to pin down El Graeco’s position in the evolutionary tree of life more precisely.

The Danger of Misinterpretation

The central question in our discussion over El Graeco’s evolutionary position is this: Is Graecopithecus more closely related to chimpanzees or to humans—that is to say, is it still a great ape or is it sufficiently evolved to be considered an early hominin? It is not easy to find a satisfactory answer to this question. There are three challenges.

The first challenge is what scientists call homoplasy, which refers to a feature that occurs independently over the course of evolution twice or even multiple times. A couple of examples will be helpful here. The trunk is a feature shared by elephants and tapirs even though these animals are not closely related. Their trunks are features that developed independently from each other. Fins, to give another example, are adaptations for life in water that arose in fish, ichthyosaurs, whales, and many other vertebrates that live in water but have no common evolutionary origin. These two examples are obvious, but with many anatomical features it is extremely difficult to recognize the parallel evolution of specific attributes, and experience has shown that homoplasy lurks everywhere, inviting mistakes in interpretation when scientists try to trace the lineage of primates.

The second challenge is caused by the close similarities between two evolutionary lines when they begin to split. Today, a wide variety of different features makes it easy to distinguish between humans and chimpanzees. However, when their last common ancestors split into two isolated populations over 7 million years ago, the external appearances of the members of both groups were still exactly the same. It was only after a geographic separation over a long period of time that the two populations developed increasingly diverse features based on random changes in their genetic material and contrasting living conditions. And only then can they be clearly distinguished from each other. But even many millions of years after a split, the members of both lines were anatomically still very similar and could possibly still interbreed. The huge differences between chimpanzees and humans today are the result of at least 7 million years of independent chimpanzee evolution plus 7 million years of independent human evolution.

The third challenge is the notoriously incomplete “fossil record.” This is what paleontologists call the sum of all existing, scientifically documented fossils sorted by where they fit chronologically, geologically, and geographically. For most mammal fossils, all we have are teeth. Only for a very few great ape fossils do we also have a skull, pelvic bones, or vertebrae, and we never find a complete spine. It is the lot of the researcher that important pieces of the puzzle are almost always missing from a dig. Also, information about soft tissue—fossilized fat layers, for instance—is rarely available. Paleontologists therefore have to be masters at completing puzzles that are missing most of their pieces.

As far as El Graeco is concerned, we are left with the following uncertainties: Graecopithecus could have evolved features that are typical of humans independently, without being part of the human lineage. That is highly improbable but still possible, and that is why we have called him a “potential early hominin.”

If Graecopithecus was an early hominin, he was only slightly different from the original ancestors of chimpanzees and, so far, we do not know what these looked like.

As we have described only two Graecopithecus fossils so far, a lower jawbone and an upper premolar, the picture we can sketch of this species is, of necessity, rather vague.

Did El Graeco Walk on Two Feet?

Probably the most important feature that unifies all the members of the human lineage and that, as far as we know, did not arise more than once is upright gait. Walking on two legs, also called bipedalism, is the revolutionary development that marks the beginning of human evolution. Proof of bipedal- ism is absolutely necessary if a fossil is to be confirmed as a hominin. The most impressive proof comes in the form of fossil footprints. These, however, are extremely rare. For the majority of fossils of early hominin species, anatomical details of the feet and legs provide valuable clues. The changes that accompanied bipedalism affected a wide range of body parts involved in locomotion: bones, musculature, sinews, and other physical features that affect biomechanics.

Even if a few monkeys and most apes can support themselves on two feet for a short while, only humans are capable of doing so for an extended period of time. Indeed, for long distances, we cannot do anything but walk on two feet. Whereas apes have to make a big effort to walk upright for just a few feet, we find it extremely difficult to walk on all fours.

The most important anatomical criteria for upright gait can be summarized as follows: Members of the human evolutionary line carry their body weight on two legs only. As the arms no longer play any role in bearing weight during locomotion, they are significantly shorter than the legs. The longer legs are the result of an elongation of the shinbones in particular. The head is balanced directly above the neck and is no longer supported by powerful neck muscles. The hole where the spinal cord enters the skull is therefore located underneath the skull instead of at the back of it. As the arms no longer restrict the chest cavity, which happens when apes walk on all fours, the human rib cage is broader than that of an ape. The shoulder blades move up and away from the sides of the body and are now located completely on the back. To cushion vertical impact, the spine is S-shaped. The pelvic girdle becomes shorter and wider, and the two broad bones at the top of the pelvis form a bowl shape. That shortens the distance between the sacrum, the triangular bone at the bottom of the spine, and the hip joint, which lends more stability to the whole hip area. The musculature of the rear end bulks up to allow bipeds to straighten their hips and stand upright. The leg muscles bulk up as well. Heavier muscles, together with longer, and therefore heavier, leg bones lower the body’s center of gravity. For a more secure stance, the thighbones angle slightly inward so the knees end up directly beneath the body’s center of gravity. 

There were also profound evolutionary changes in the anatomy of the feet as great apes evolved into humans. Hominin feet are no longer appendages used for grasping things. They now facilitate a stable stance on two legs and, even more important, efficient, swift, balanced, forward locomotion. In contrast to the big toe of apes, the human big toe is no longer splayed out to the side. It is oriented forward, parallel to the other toes, which are considerably shorter in comparison. At the base of the enormous big toe, the foot developed a ball, which has a new, extremely important function in the sequence of motion. The big toe, as the last toe in contact with the ground, works in concert with the ball of the foot to propel the body forward.

 

In later stages of evolution, early humans such as Homo erectus developed an arch to the foot to absorb the shock of repeated, forceful contact with the ground. That was mostly necessary because the lifestyles of early humans meant the ability to run long distances became much more important than it had been for early hominins (see Chapter 20).

Even though the list of features required for upright gait is long, there are significant challenges to recognizing the early stages of bipedalism. Even with well-documented skeletons, such as that of Lucy, there was heated scientific debate. The controversy mostly centered around whether this species of early hominin could be said to have developed a modern style of bipedalism or whether Lucy still spent most of her time living in trees.

In many finds that mark the beginning of the human ancestral line, crucial anatomical regions are missing. In others, a few characteristic features have been found, such as the fragment of a shinbone (Australopithecus anamensis), a section of thighbone (Orrorin tugenensis), or a small foot bone (Ardipithecus kadabba); however, because of the complex interaction of many regions of the body in upright gait, such sparse evidence is often not sufficient for us to be able to say definitively whether these species spent most of their time walking on two legs or not.

As mentioned, the surest evidence of upright gait is a fossil footprint. But such a discovery, one of the most price- less historical records a paleontologist could find, would be an exceedingly rare stroke of luck. Until recently, the fossil footprints of early hominins had been found only once: 3.6-million-year-old footprints from Laetoli in Tanzania that definitively ended the debate about Lucy’s upright gait. Today, more mysterious tracks of a biped, dating back millions of years, have emerged—this time on the island of Crete.


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