The remains of a tiny and hitherto unknown species of human that lived as recently as 13,000 years ago have been discovered on an Indonesian island.
The discovery has been heralded as the most important palaeoanthropological find for 50 years, and has radically altered the accepted picture of human evolution.
The skull and bones of one adult female, and fragments from up to six other specimens, were found in the Liang Bua limestone caves on Flores Island, which lies at the eastern tip of Java.
The female skeleton, known as LB1 - or by the nickname "Ebu" - has been assigned to a new species within the genus Homo - Homo floresiensis. Examination of the remains shows members of the species stood just 1 metre tall and had a brain no bigger than a grapefruit.
A handful of stone tools from the same period were also found in the caves, along with the bones and teeth of several dwarf stegodons, an ancestor of the modern elephant. Other animal remains, including rats, bats and fish, show signs that they were cooked around the time H. floresiensis inhabited in the caves.
Instead of following a simple evolutionary path culminating in modern humans - Homo sapiens - the discovery of LB1 suggests early humans branched into many more forms than previously thought - some of which survived until very recently. The find also shows that small-brained humans could evolve without losing much of their intelligence.
"It is literally jaw-dropping," says Bernard Wood, an anthropologist at George Washington University in Washington DC, US. "The implications for the evolution of the [human] brain are among the most interesting."
"It raises the whole issue of what it is to be human, or a member of the genus Homo," adds Chris Stringer at the Natural History Museum in London, UK. "And shows how little we really know about human evolution."
Homo sapiens are thought to have colonised Flores island between 55,000 and 35,000 years ago, meaning they must have lived alongside H. floresiensis for tens of thousands of years.
Indonesian archaeologists have dug at the Liang Bua cave site for the past 20 years, so at first the small skull misled the excavation team.
"We thought the skull and the mandible was from a child," says excavation site director Thomas Sutikna of the Indonesian Centre for Archaeology, who was there when the discovery was made in September 2003. "But after a week, we checked the teeth and saw that they were already worn, and that the molars had erupted, so she was more than 20 years old."
The authenticity of the remains has been confirmed by three-dimensional X-ray images which revealed the internal structure of the skull of LB1, something that would be virtually impossible to fake.
Accelerator mass spectrometry dating suggests that LB1's remains are 18,000 years old. But New Scientist has learned that some bone fragments could be as young as 13,000 years old. The oldest remains from the site are 78,000 and 94,000 years old, respectively.
This means H. floresiensis survived well beyond the last Neanderthals, which are thought to have disappeared from Europe and western Asian about 28,000 years ago.
"The most remarkable thing is that there was a time, not so long ago, when two very different human species walked the planet," says Peter Brown of the University of New England, Australia, who led the team that made the discovery.
H. floresiensis is thought to have descended from a population that became marooned on Flores during the last few hundred thousand years. Its tiny size was probably an adaptation to isolated island conditions, where a low calorie diet and the lack of large predators made smaller physical characteristics advantageous.
There is no evidence the Flores was ever linked by land to other parts of south-east Asia, so the ancestors of this species might have used boats or rafts to reach it. But the fact this population evidently became isolated suggests that they may have relied on temporary land links to first reach the island.
What caused the demise of H. floresiensis is unknown. It is possible that they were out-competed for food and other resources by H. sapiens or that they were wiped out by a volcanic eruption about 12,000 years ago.
Journal reference: Nature (vol. 431, p. 1055, p. 1087)
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