How the skull of the oldest ancestor known to mankind changes our understanding of evolution

The recent discovery of a 3.8 million year old skull (skull without the lower jaw) is currently the most hot topic of conversation among paleoanthropologists. But there are fossils all the time, so why is the skull of this little old man so important? It turns out that the discovery changes our vision of how the first species of hominids evolved – and led to humans. To understand how, let's start from the beginning.

In 1995, researchers in Kenya found several partial jaws, isolated teeth and limb bones from 4.2 to 3.9 months old, and attributed them to a brand new species: Australopithecus anamensis. All of these fossils have been found in sediments associated with an ancient lake – "anam", which means lake in the local language. A number of additional specimens were then found in Ethiopia, belonging to the same species.

The primitive characteristics of A. anamensis have led to the widespread opinion that this species is the ancestor of Australopithecus afarensis, a young hominid from Tanzania, Ethiopia and possibly Kenya, 3.8 to 3 million years old. The most emblematic fossil of A. afarensis is probably the partial skeleton known as Lucy, who has long been considered the oldest known human ancestor.

The recently discovered skull, nicknamed "MRD" after its MRD-VP-1/1 collection number, has many similarities with the existing model. A. anamensis specimens, and was therefore assigned to this species. However, the MRD skull was sufficiently intact to allow scientists to analyze for the first time the complete face and the puzzle, and examine the parts of the skull that were still missing in the fossil record. A. anamensis.

The authors have discovered several new morphological features in the MRD skull that are classically considered characteristic of younger species of the human lineage. The depth of the mouth, for example, exceeds that of all others A. anamensis and A. afarensis specimens, and even is among the deepest palaces of later Australopithecus species. This calls into question the long-standing and widespread view that Lucy's species have gradually evolved A. anamensis without branching the evolutionary line – a process called anagenesis.

Since these modern features were already present in older species, the most likely scenario is that Lucy's species are formed by evolutionarily divergent A. anamensis – a process known as cladogenesis. We do not know when exactly A. afarensis diverged. Another evidence of cladogenesis comes from a 3.9 million year old frontal bone of Ethiopia (part of the forehead) discovered in 1981. Its shape is different from that of MRM, which suggests that this fossil probably belongs to A. afarensis.

If this is the case, we need to review the chronology of human evolution, with A. anamensis 4.2 to 3.8 million years ago, and A. afarensis from 3.9 to 3 million years old. This would imply that the two species overlap for at least 100,000 years, making it impossible A. afarensis have evolved gradually from a single ancestral group. In fact, it is becoming increasingly evident that most species in our evolutionary line have probably evolved by branching from existing groups.

The human line

The new discovery also challenges the idea that Lucy's species is the ancestor of all future generations. Australopithecus the hominins, which eventually led to the man.

A vertically straight curvature and a sharp increase in the cheekbone is traditionally considered a relatively modern feature. C & # 39; was present in Australopithecus africanus (3.7m-2.1m years ago) in southern Africa, considered by some as a direct ancestor of the Homo lineage) and in Paranthropus (2.7 to 1.2 million years ago from southern and eastern Africa, not directly on our evolutionary line).

The opposite condition – a low and arched cheekbone – is considered primitive and is shared between A. afarensis, Ardipithecus ramidus (There are between 4.3m and 4.5m years from Ethiopia, a primitive hominin more like a monkey) and African monkeys.

The crest of the MRD skull, which is surprisingly modern, now defies this view. It also opens the possibility that the long-standing idea of A. afarensis as the ancestor of all later Australopithecus groups may have been wrong, and that place A. anamensis is the ancestor of these younger species. The first hominin, direct ancestor of the man, remains an unanswered question.

It is clear that this latest discovery has brought new information about our evolutionary past, but has also increased the complexity of the relationship between ancient hominins. The Middle Pliocene (between 5.3 and 2.6 million years) is full of multiple, contemporary and geographically extensive species.

Clarifying the relationships between these species, confidently characterizing their morphology and deciphering the complex and complex history of hominin evolution is not an easy task. The specimens of each new site capture a different point in the evolutionary trajectory, but it is difficult to convert these results into stable and reliable branches on an evolving tree.

More specimens from epochs and geographical locations that are currently underrepresented in the fossil record could help solve these problems, but could also upset everything we know.

The discoveries made around the world over the past decade have led to a complete rethinking of our evolutionary past. This shows that the new fossils do not always corroborate the existing hypotheses and that we must be ready to modify our points of view and formulate new theories on the basis of the available evidence.