The versatility that helped humans take over the world emerged very early in our evolutionary history, according to sediments and stone tools from Olduvai Gorge in Tanzania.

Olduvai has provided some of the oldest known tools and fossils from our genus, Homo . A recent study lines that evidence up with environmental clues buried in the sediment. The results suggest that our early relatives were equipped to adapt to new environments by around 2 million years ago.

That seems to have been a key ability that allowed our relatives to go global. By 1.7 million years ago, an early human relative called Homo erectus had spread beyond Africa and throughout most of Asia, as far as Indonesia. They had reached western Europe by 1.2 million years ago. Along their travels, the hominins encountered environments very different from the ones their ancestors had evolved in, like the tropical forests of Indonesia and the arid steppes of central Asia.

Comparison of Modern Human and Neanderthal skulls from the Cleveland Museum of Natural History. (credit: hairymuseummatt )

We know that Neanderthals left their mark behind in the DNA of many modern humans, but that exchange worked both ways. The groups of Neanderthals our species met in Eurasia around 45,000 years ago already carried some Homo sapiens genes as souvenirs of much earlier encounters. A recent study suggests that those early encounters allowed the Homo sapiens version of the Y chromosome to completely replace the original Neanderthal one sometime between 370,000 and 100,000 years ago.

Evolutionary geneticists Martin Petr, Janet Kelso, and their colleagues used a new method to sequence Y-chromosome DNA from two Denisovans and three Neanderthals from sites in France, Russia, and Spain (all three lived 38,000 to 53,000 years ago). The oldest Neanderthal genomes in Eurasia have Y chromosomes that look much more like those of Denisovans. Later Neanderthals, however, have Y chromosomes that look more like those of us humans.

Gene flow is a two-way street

Tens of thousands of years ago, our species shared the world with at least two other hominins. The tools, beads, and art they left behind hint that these other humans were probably a lot like us. And we were definitely all alike enough to have, apparently, a bit of sex.