ow did we modern humans (Homo sapiens) evolve?

These GEITP pages have often examined the latest advances in our understanding of how we modern humans (Homo sapiens) evolved. During the last three decades, our understanding has advanced greatly. Most research has supported the theory that Homo sapiens had originated in Africa “no more than ~200,000 years ago (Ya)”, but the latest discoveries suggest that the events were more complex than previously thought (this is common in virtually every field of science). The data confirm interbreeding between Homo sapiens and other hominin species. The data also provide evidence for Homo sapiens in Morocco as early as 300,000 Ya; the data further indicate incremental changes in shape of the cranium (skull) of Homo sapiens. Although cumulative evidence still suggests that all modern humans descended from African Homo sapiens populations –– which had replaced local populations of earlier archaic hominins, models of modern human origins now must include that “substantial interactions” with those populations (Homo neaderthalensis & Homo denisova) had occurred before they became extinct [see the tree diagram on p 1297 of attached editorial].

Although today’s humans vary in traits such as body size, shape, facial structure and skin color, we clearly belong to a single species, Homo sapiens –– which are characterized by shared features such as narrow pelvis, large brain housed in a globular braincase, and decreased size of teeth and surrounding skeletal architecture. These traits (phenotypes) distinguish modern humans from other (now-extinct) members of the genus Homo (e.g. the Neanderthals in western Eurasia and the Denisovans in eastern Eurasia). This excellent editorial [attached] describes chronologically how a 1987 study, using mitochondrial DNA from modern humans, indicated a recent and exclusively African origin for modern humans. In the following years, fossil and genetic data, combined, then supported further the recent African origin (RAO) for our species.

The RAO Model postulates that, by 60,000 Ya, the shared features of modern humans had evolved in southeast Africa and, via population dispersals (i.e. the Great Human Diaspora), began to spread from there across the world. Some have opposed this “single-origin” view, and the narrow definition of Homo sapiens to exclude fossil humans such as the Neanderthals. In recent years, however –– new fossil discoveries, the growth of ancient DNA technology, and improved radiocarbon-dating techniques (of rocks in which fossils with DNA are embedded) have raised questions about whether “the RAO theory of Homo sapiens evolution” needs to be completely revised.

We now know that ~6% of the modern human genome comprises Neaderthal alleles (two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome), and some populations (e.g. Oceanians) have genomes carrying substantial amounts of Denisovan alleles. So, there is growing evidence for a longer-term “coexistence” (i.e. admixture, messing-around, hanky-panky, etc.) of Homo sapiens and other lineages outside of Africa –– consistent with the Assimilation Model. The accumulated evidence still points to the evolution of shared anatomical features of Homo sapiens as an African phenomenon. How the ancestral populations interacted within Africa, currently looks unclear; genomes have not been successfully reconstructed from African fossils that are older than about 15,000 Ya. However, if such data become available, they will hopefully clarify many of the remaining uncertainties. 🙂

Science 22 June 2o18; 360: 1296–1298

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