Why is there only one human species?
Not so very long ago, we shared this planet with several other species of human, all of them clever, resourceful and excellent hunters, so why did only Homo sapiens survive?
Huge debates rage about human origins, but the broad consensus among scientists is that all the different species of human that have ever existed were descended from ape-like creatures that walked upright in Africa more than six million years ago.
These creatures had many descendants, most of which became extinct, but the first creature we would recognise as human first appeared in Africa two million years ago.
Known as Homo ergaster, they made tools and were proficient hunters. Their bones suggest they would have been powerful runners, capable of speeds that would rival a modern Olympic athlete.
H. ergaster seems to have evolved during a long period of terrible drought which dried out tropical rainforests and created vast deserts.
This human species was equipped to cope with heat. They would have been smooth and largely hairless, allowing them to sweat more efficiently. H. ergaster could also travel and hunt in the middle of the day, when most animals rest.
And we know that they travelled long distances because they did not stay in Africa. A hungry meat eater, ergaster became the first human to leave Africa and colonise Asia.
Here, in a new and lush environment, they evolved and got a new name, Homo erectus.
Archaeological records show they spread over an area ranging from Turkey to China, but the population may not have been that large.
"These were small groups of hunters and gatherers," says Professor Chris Stringer, an anthropologist at the Natural History Museum.
"These are people that are being very mobile, in open country, to get to their food ahead of the competition. So in that sense, they're very like us in terms of their overall body shape and body build."
Recent findings suggest that Homo sapiens also left Africa, around 120,000 years ago.
We travelled in small numbers, possibly no more than 100 in the first wave. Then we spread out, with some eventually reaching Europe, then occupied by the Neanderthals, while others moved east until they reached India. There is archaeological evidence that they arrived just in time for a truly cataclysmic event.
About 74,000 years ago Mount Toba, a volcano in South East Asia erupted in spectacular fashion, the biggest explosion in the last two million years. Because of its magnitude it is classed as a supervolcanic eruption.
The volcano spewed enough sulphur into the atmosphere to lower world temperatures by several degrees and enough molten rock to cover an area the size of Britain to a depth of 10 metres.
It also produced vast amounts of ash. Driven by the winds, clouds of white Toba ash covered huge swathes of Asia, including much of the Indian subcontinent. It can still be found today.
Whether it was the effects of Toba, or the arrival of modern humans, the eruption marks the high tide of erectus' occupation of Asia.
Over the next 40,000 years they were slowly driven out, probably by a combination of climate change and the effects of being out-competed for scarce food by the spread of modern humans.
Yet 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.
"If that gap were populated by other hominids, we'd see that gap as not so much a gulf but rather a continuum with steps on the way. We'd still think of ourselves as special, but maybe not so special - a little dose of humility wouldn't hurt.
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