The first humans
July 23, 2017
The discovery in Jebel Irhoud, Morocco of human fossils with modern facial features, similar to ours, has been a wonderful surprise, even outside the world of anthropology. The discoveries have been published in the journal Nature by Jean-Jacques Hublin and collaborators. The fossils are associated with tools from the Middle Stone Age, the technique immediately preceding the Upper Pleistocene. The surprise is due to the age of the tools and of the fossils, somewhat older than 300,000 years. The oldest Homo sapiens previously known, from Florisbad, South Africa, are more recent. The fossil tooth found in this deposit had been dated, using the electron spin resonance (ESR) technique, at 295,000 ± 35,000 years.
Commentaries concerning the new discoveries of Jebel Irhoud highlight two significant facts. First, human fossils from the same Moroccan deposit had been known since the 1960s but the age attributed to them was much more recent. The second and most significant point concerns the age of our own species. Molecular biology data (mitochondrial as well as nuclear) show that Neanderthals and modern humans diverged about half a million years ago. Why is it that all available evidence about fossils similar to us is much more recent?
The answer emerges from how we come to identify the divergence between two lineages. Most likely the Neanderthals evolved from ancestors, which were also our own, who left Africa about half a million years ago. But thereafter, the lineages of the Neanderthals and of modern humans evolved independently, the first in Europe and ours in Africa. Although at later times, after the Cro-Magnons migrated out of Africa, the two lineages would occasionally intermix in the Near East and in Europe.
After about 500,000 years ago, the lineage that would eventually give rise to Homo sapiens evolved in Africa. Gradually, of course, but following a pattern known as “phyletic” evolution, namely without branchings. The quandary that arises in a process of gradual change within a single lineage, is that it becomes impossible to determine a precise time when a new species arises, such as our own. A similar quandary arises if we want to determine when an adolescent becomes an adult: the change is gradual; there is not a precise moment at which the transition occurs. We are the outcome of a gradual process of change, expanded over half a million years, which eventually yielded our current features.
Determining when this actually happened will depend on new fossils yet to be discovered and what they tell us about their evolutionary characteristics. Thanks to the fossils from Jebel Irhoud we now know that 350,000 years ago there were already humans with facial features similar to ours, although with somewhat different configuration of the skull. Taking into account the considerable phenotypic variation of our species, both Jebel Irhoud and Florisbad indicate that the steady process towards modern humans had reached an evolutionary stage similar to ours at least 300,000 years ago.
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