Fossils found in a South African cave might belong to a new species of early humans that holds a unique place on the evolutionary road to modern man.
On a 2008 mapping expedition in Malapa, South Africa, American paleoanthropologist Lee Berger made a remarkable discovery: the fossilized bones of a human-like woman and a boy buried in cave sediments. The two - possibly a mother and her son - lived almost two million years ago, about the time anthropologists believe the first modern humans were beginning to walk the Earth.
The new species, one of a family of non-ape human ancestors known as hominins is called Australopithecus sediba.
Berger says the age of the find is significant.
"The most accurate date that probably has ever been produced for an early hominin site in Africa at 1.977 million years, placing Australopithecus sediba squarely in the potential for being the best candidate for the ancestor leading to the emergence of the genus homo," says Berger.
The partial skeletons reveal that the hominins had an ape-sized brain but human-shaped hips and pelvis. They walked upright on long legs, but also climbed trees with even longer, ape-like arms.
Report co-author Tracy Kivell, with the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, notes in her study of the adult female's hand that the fossils display a mix of primitive features and modern traits.
"Just like the rest of the skeleton," Kivell says, "the Sediba hand revealed a surprising mix of features that we wouldn't have predicted could exist in the same hand."
Kivell says the hand, which is the most complete in the hominin record, is very modern-looking, with a long thumb and short somewhat curved fingers - a sign of precision gripping.
"By precision grips, I just mean when we move the pad of our thumb towards the pads of our fingers like when we're holding a pen or using a key."
And, since primitive stone tools are evident in the archaeological record more than a half-million years before Sediba's time, Kivell says it's possible these hominins used their versatile hands to make tools.
"We think that with this long thumb and short fingers in Sediba that it would have been capable of making those finer movements and manipulating objects in a way that's similar to humans and later homo."
Researchers also used a powerful x-ray scanner to create high resolution images of the hominin's brain case. Kristian Carlson, of the University of Witwatersrand, says that while the brain is ape-sized, the expanded region behind where the olfactory, or smell sensors, are located suggests Sediba's more advanced thinking ability.
"We can't talk about interconnections of neurons, but what we can say - and maybe that's very little - is that this expansion could be related to the initial development of these sorts of activities and those are information in coding and retrieval, relational reasoning and multitasking abilities."
The fossils also include one of the more complete pelvises every discovered and a foot with evidence of a human-like arch and well-defined Achilles tendon. The heel and shin bone appear to be more ape-like.
With its mosaic of traits, the Sediba find has sparked a debate over exactly where it belongs on the human family tree.
The consensus, as reported in the Journal Science, is that Australopithecus sediba represents a transitional species to the genus Homo, perhaps as ancestor to Homo erectus, which leads to our own species, Homo sapiens.
Although Sediba's discovery adds an important new chapter to the human story, much more work remains. Team leader Berger says it will take continued finds of quality fossils to fill the remaining gaps in our evolutionary history.