why is there only one human species?

bbc |  Homo erectus was slightly bigger and more powerful than Homo sapiens, so why did we thrive when they did not? The most obvious answer is that we had bigger brains - but it turns out that what matters is not overall brain size but the areas where the brain is larger. 

"The Homo erectus brain did not devote a lot of space to the part of the brain that controls language and speech," said John Shea, professor of palaeoanthropology at Stony Brook University in New York.

"One of the crucial elements of Homo sapiens' adaptations is that it combines complex planning, developed in the front of the brain, with language and the ability to spread new ideas from one individual to another.. "

Planning, communication and even trade led, among other things, to the development of better tools and weapons which spread rapidly across the population.

The fossil records suggest that H. erectus went on making the same basic hand axe for more than a million years.

Our ancestors, by contrast, created smaller, more sophisticated weapons, like a spear, which can be thrown, with obvious advantages when it comes to hunting and to fighting.

The same advantages helped Homo sapiens outcompete another rival human, the Neanderthals, who died out about 30,000 years ago as the Ice Age limited available food supplies. 

"Even 100,000 years ago, we've still got several human species on Earth and that's strange for us. We're the only survivors of all of those great evolutionary experiments in how to be human," says Stringer.

H. erectus hung on in Asia until 30,000 years ago. Although they went extinct, they appear to have left descendants on the island of Flores in Indonesia.

These humans, Homo floresiensis, also known as "Hobbits", survived until around 12,000 years ago. And then they went, leaving us as the last human species on the planet. 

"There's such a huge gulf between ourselves and our nearest primate relatives, gorillas, chimpanzees and bonobos," said Dr Shea.