Mixing with extinct humans: what’s the evidence?
The Neanderthals, or hom*o neanderthalis, were an extinct species of human that was widely distributed across ice-age Europe and Western Asia. They had slightly different shaped heads to modern humans, including having a receding forehead and prominent brow ridges.
Since discovering the first Neanderthal fossil in 1856 near Düsseldorf, Germany, researchers have been striving to uncover the position ofhom*o neanderthalisin modern human evolution. hom*o neanderthalisappeared in Europe about 250,000 years ago and spread into the Near East and Central Asia. They disappeared from the fossil record about 28,000 years ago – likely caused by competition from modern humans.
Although most research supports the ‘out of Africa’ model, some genomic research supports the multi-regional model. This second model suggests that early human or human-like species interbred with each other in different parts of the world, eventually leading to the hom*o sapiens we are today.
Sequencing the Neanderthal genome
DNA can survive in bone long after an animal dies. Over time, the animal’s DNA becomes contaminated with microbes and their DNA – but it is still possible to decontaminate and extract DNA from ancient bones, up to several thousand years old.
In 2010, scientists sequenced the DNA of an entire Neanderthal genome. This was improved upon in 2013, when a more refined Neanderthal genome sequence was obtained from a 50,000-year-old Neanderthal toe bone. As with the human genome sequence, the Neanderthal genome sequence was made available online for free.
Analysing the Neanderthal genome revealed clues as to why the species went extinct. The toe bone came from someone with two X chromosomes, which were both similar in sequence. This suggests their parents were closely related. Inbreeding is generally bad for the fitness of a species, making it more susceptible to disease and illness.
If inbreeding was common in the Neanderthals, this could have led to reduced genetic variation and their eventual extinction.
Introducing the Denisovans
While investigating the Neanderthal toe bone, scientists identified a finger bone and, upon analysing its genome, identified another extinct human-like species called ‘Denisovan’ – named after the Siberian cave in which the finger was found.
Analysing the Denisovan and Neanderthal genomes suggested that these species interbred with each other. Additionally, early modern humans interbred with these now extinct species as they journeyed along coastlines and over mountains.
In favour of the ‘out of Africa’ model
When comparing genomes, modern human genomes resemble each other more than any resemble the Neanderthal genome. Additionally, some Neanderthal DNA is similar to human genomes from people of European and Asian origin – but these similarities are not seen in DNA from people of African origin.
This suggests that modern humans evolved in Africa and then expanded out into Asia and Europe, where Neanderthals lived – and where a degree of inbreeding between Neanderthals and earlyhom*o sapiens then took place. For example, one study suggests this inbreeding took place between 37,000 and 85,000 years ago, resulting in up to around 2% of Neanderthal-derived DNA in people not of African descent.
