Sapiens: a brief History of Humankind

Map 1
. 
Homo sapiens
conquers the globe
.
A lot hinges on this debate. From an evolutionary perspective, 70,000 years is a
relatively short interval. If the Replacement Theory is correct, all living humans
have roughly the same genetic baggage, and racial distinctions among them are

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negligible. But if the Interbreeding Theory is right, there might well be genetic
di erences between Africans, Europeans and Asians that go back hundreds of
thousands of years. This is political dynamite, which could provide material for
explosive racial theories.
In recent decades the Replacement Theory has been the common wisdom in the
eld. It had rmer archaeological backing, and was more politically correct
(scientists had no desire to open up the Pandora’s box of racism by claiming
signi cant genetic diversity among modern human populations). But that ended
in 2010, when the results of a four-year e ort to map the Neanderthal genome
were published. Geneticists were able to collect enough intact Neanderthal DNA
from fossils to make a broad comparison between it and the DNA of contemporary
humans. The results stunned the scientific community.
It turned out that 1–4 per cent of the unique human DNA of modern populations
in the Middle East and Europe is Neanderthal DNA. That’s not a huge amount, but
it’s signi cant. A second shock came several months later, when DNA extracted
from the fossilised nger from Denisova was mapped. The results proved that up
to 6 per cent of the unique human DNA of modern Melanesians and Aboriginal
Australians is Denisovan DNA.
If these results are valid – and it’s important to keep in mind that further
research is under way and may either reinforce or modify these conclusions – the
Interbreeders got at least some things right. But that doesn’t mean that the
Replacement Theory is completely wrong. Since Neanderthals and Denisovans
contributed only a small amount of DNA to our present-day genome, it is
impossible to speak of a ‘merger’ between Sapiens and other human species.
Although di erences between them were not large enough to completely prevent
fertile intercourse, they were sufficient to make such contacts very rare.
How then should we understand the biological relatedness of Sapiens,
Neanderthals and Denisovans? Clearly, they were not completely di erent species
like horses and donkeys. On the other hand, they were not just di erent
populations of the same species, like bulldogs and spaniels. Biological reality is
not black and white. There are also important grey areas. Every two species that
evolved from a common ancestor, such as horses and donkeys, were at one time
just two populations of the same species, like bulldogs and spaniels. There must
have been a point when the two populations were already quite di erent from
one another, but still capable on rare occasions of having sex and producing
fertile o spring. Then another mutation severed this last connecting thread, and
they went their separate evolutionary ways.
It seems that about 50,000 years ago, Sapiens, Neanderthals and Denisovans
were at that borderline point. They were almost, but not quite, entirely separate
species. As we shall see in the next chapter, Sapiens were already very di erent

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from Neanderthals and Denisovans not only in their genetic code and physical
traits, but also in their cognitive and social abilities, yet it appears it was still just
possible, on rare occasions, for a Sapiens and a Neanderthal to produce a fertile
o spring. So the populations did not merge, but a few lucky Neanderthal genes
did hitch a ride on the Sapiens Express. It is unsettling – and perhaps thrilling – to
think that we Sapiens could at one time have sex with an animal from a di erent
species, and produce children together.