How human are humans?
Evidence has piled up over years now that humans came from Africa: a lovely interactive map in Nature gives a summary of the history and part of the evidence for it, and can be read together with a special feature on the subject. Nature carries an article by Chris Stringer on the evidence that Homo sapiens interbred with other closely related species:
For the past 25 years, there has been a fierce debate about whether our species evolved from distinct ancestors who were spread across the Old World (multiregional evolution) or whether, as I have advocated, we evolved recently in only one area — Africa (recent African origin). I have argued that the physical traits of modern humans can be divided into two sets of features, which have originated through different mechanisms and over different time scales. The first set includes features shared by all living humans — such as a high and rounded skull, small brow ridge and a bony chin on the lower jaw — that make it clear a skull belongs to H. sapiens, and not an ancient form. These traits had evolved in Africa by 100,000 years ago, and were exported from there in an exodus that began about 60,000 years ago.
The second set of traits belonging to modern humans include the regional or ‘racial’ features that differentiate human populations, such as facial shape, form of the eyelids, type of hair, skin pigmentation and physique. I have argued that these regional features were added as people spread in small numbers after the shared modern human template had evolved in Africa.
That dispersal event potentially brought early modern humans into the realms of other hominin populations living outside Africa, including the Neanderthals in western Eurasia, and Homo floresiensis and the ancient species Homo erectus in Indonesia. The first DNA to be successfully recovered from Neanderthal fossils seemed to support the view that the Neanderthals represented a lineage and species that was separate from all recent humans. However, increasingly complete genomic reconstructions from Neanderthal fossils and from a newly characterized group called Denisovans — so far known only from a single cave in southern Siberia — have shown these hominins to be related populations that descended from the earlier species Homo heidelbergensis, which also gave rise to H. sapiens (see ‘A winding path’).
Comparative genomic studies have revealed elements of DNA that are unique to each of the three groups (recent humans, Neanderthals and Denisovans), some of which could be expressed in the phenotype, and that may be related to such things as the physiology of the brain, skin, skeleton and even sperm. But the biggest surprise for many researchers was the evidence from whole genome scans that modern humans living outside Africa each carry about 2.5% of their DNA from Neanderthals; furthermore, people living today in Australia and New Guinea (Australasians) carry about 5% of Denisovan DNA.
The most likely explanation is interbreeding, such as when modern humans emerged from Africa into Eurasia about 60,000 years ago and met Neanderthals, and when the ancestors of Australasians met some Denisovans. Scientists are continuing to study these archaic genomes in detail, and are finding clear variation within and between non-African people in the amount and kind of archaic DNA that survives in their genomes today. In short, people living outside Africa carry different quantities, and distinct remnants, of archaic DNA from those interbreeding events.
The reason this article is more interesting than the newspaper reports that will inevitably follow is that it squarely faces a question that each of us will have, and tries to answer it:
This evidence of interbreeding between groups that palaeontologists call separate species raises two crucial questions. First, given that most of us learned at school that species don’t interbreed, should we change the definition we use for a species? Or should we remove the taxonomic separations erected purely from the morphology of fossils, and sink H. heidelbergensis, Neanderthals and Denisovans into H. sapiens? And second, how does the evidence of interbreeding affect our concept of modern humans, when the genomes of people today apparently contain differing levels of archaic genes?
In my view, the evidence that H. sapiens interbred with archaic humans does not yet require a merging of these close relatives into a single expanded concept of H. sapiens. Doing so would produce a species that had a range of morphological variation several times that found in humans today, or in other existing primate species. These human lineages were distinct enough to build up well-differentiated genotypes and phenotypes (although we know little of the Denisovan phenotype so far) — even the inner ear bones of a Neanderthal are readily distinguishable from those of a modern human. Furthermore, many closely related species of primates undergo limited interbreeding, including among our close ape and monkey relatives. So, for pragmatic reasons, I would retain these species categories while recognizing that this does not imply complete reproductive isolation.
The article also raises and answers a point that may disturb us:
Terms such as ‘archaic’ and ‘primitive’ may be considered objective when used by palaeontologists, but they can be pejorative in common parlance. If researchers want to continue the progress recently made in studying the origins of modern human variation, they will need to think long and hard about their aims, and the lexicon they use.
One thing should be reiterated: all living humans are members of the extant species H. sapiens and, by definition, all must equally be modern humans. The majority of our genes (>90%) derives from our common African heritage, and this should take precedence over the minor amount of DNA that is different — however and whenever it was acquired.