[LINK] "Q&A: Who is H. sapiens really, and how do we know?"

[rfmcdonald]

lux_apollo is to be thanked for linking to this question-and-answer format essay by scientists Mason Liang and Rasmus Nielsen explaining what evidence there is for non-human but hominid ancestry (Neanderthals, Denisova, Homo erectus et cetera) in the modern human gene pool.

Is it true that modern humans have Neanderthals and other archaic species in their direct ancestry?

According to two recently published papers by Green et al. and Reich et al., the answer to this question is yes. Human genomes are in part composed of DNA from other archaic hominin species that traditionally have not been counted among our ancestors, although the proportion of archaic DNA in the genome depends on your ethnicity. On the basis of analyses of ancient DNA, Green et al. report that, on average, 1 to 5% of the genomes of non-African individuals are descended from a Neanderthal, and Reich et al. report that 4 to 6% of the genomes of Melanesians are derived from a newly discovered archaic hominin population dubbed the Denisovans. Denisovans and Neanderthals are the only archaic species investigated so far, but future investigations may reveal contributions of DNA from other species, perhaps even from species that have never been characterized well morphologically.

What is an archaic hominin, exactly?

Hominins are humans and their closely related extinct ancestors. Denisovans and Neanderthals were hominins that last lived approximately 30,000 years ago. Neanderthal fossils were first found in 1856, in the Neander Valley, which lends its name to the species. Since then, specimens have been found in a wide geographical range, including the Middle East, Central Asia, and Western and Central Europe. To date, the only discovered Denisovan remains are the finger bone and two teeth discovered in Denisova Cave in Siberia. On the basis of genetic analysis of the finger bone, Reich et al. conclude that Denisovans represent a deeply diverged population distinct from other Neanderthals. Whether Neanderthals and Denisovans comprise separate species is probably mainly an issue of semantics and, in any case, cannot be answered without additional Denisovan samples.

How does this fit with current theories of human origins?

The question of human origins has intrigued scientists ever since Darwin first proposed the theory of evolution. Historically, most of the debate has focused on two competing hypotheses: the out of Africa (OOA) theory and the multi-regional theory. The OOA hypothesis posits that anatomically modern humans first evolved in Africa 200,000 to 150,000 years ago and then migrated out of Africa 100,000 to 60,000 years ago, displacing other archaic hominins, and giving rise to all current human populations. The multi-regional theory suggests that archaic hominins spread out of Africa much earlier, and that humans then evolved from this Eurasia-wide population, with some degree of interbreeding, and thus gene flow, among individuals from different populations being responsible for the degree of genetic differentiation between populations we currently observe. Mitochondrial (mt) DNA data first reported in 1987 and subsequent analyses of autosomal DNA seemed to support the OOA hypothesis.

However, even before the publication of the Neanderthal genome, analyses of modern human DNA from different geographic sources by Jeffrey Wall and others had suggested that, contrary to the earlier consensus, anatomically modern humans evolved in Africa recently, but admixed with endemic archaic hominids - Neanderthals, Denisovans, or even Homo erectus - as they spread throughout the world, and that ancestral admixture may be much more common than previously thought.

The most interesting issue that Liang and Nielsen raise is the possibility that other distinctively hominin populations not yet identified may have left descendants. And, of course, there are all the different hominin populations which didn't leave descendants ...