The two partial skeletons of an adult female and child were found in miners' debris in the Cradle of Humankind World Heritage Site in 2008 by Professor Lee Berger from South Africa's University of the Witwatersrand.

The species - named Australopithecus sediba - has features of both earlier bipedal apes and more recent species of early Homo, the scientists claim. The anatomy and age of the remains are described in two papers in the latest issue of the prestigious journal Science.

An international team of more than 60 scientists was involved in the identification and dating of the skeletons, including Dr Andy Herries from the University of New South Wales, Dr Robyn Pickering from the University of Melbourne and Dr Paul Dirks from James Cook University.

"The newly documented species appears to be a very good transitional form, maybe the best yet found, between Australopithecines and early members of the genus Homo," Professor Berger said.

The name sediba, meaning "natural spring" in the Sesotho language, seemed appropriate for a species that might be the point from which the genus Homo arises, he said.

The Australian scientists dated the fossil-bearing layers to between 1.95 and 1.78 million years-old. The remains were dated at around 1.95 million years, placing the new species at a transition point in our evolutionary story from small brained bipedal apes to larger brained human ancestors.

Dr Dirks, a geologist, led the team studying the context of the fossils. The task of dating the remains fell to Dr Herries, a palaeoanthropologist from UNSW's School of Medical Sciences and Dr Pickering, from the University of Melbourne, both world leaders in the science of geochronology. The trio used advanced uranium-lead radiometric dating combined with palaeomagnetism - the study of the effects of changes in the Earth's magnetic field on the alignment of minerals in cave deposits - to establish the fossils' age.

The creatures most likely fell into a 50 metre-deep cave soon after the start of a period of 'normal' magnetic polarity around 1.95 million years ago, the scientists found.

"Until recently it was impossible to get precise dates for the South African cave sites, but with the development of new techniques we are beginning to understand the relationships of the various species of early human to each other," said Dr Herries.

"This is a period of major climatic change and increasing aridity in Africa, when a number of different early human species occur that are potential ancestors to Homo erectus, each adapting to these changes in different ways. Australopithecus sediba appears to have traits of both the earlier species, Australopithecus africanus, and a later species, Homo erectus," he said.

Traits of modern humans
Information from the skeletons shows Au. sediba was an upright walker, around 1.27 meters tall, and shared many of the physical traits of the earliest humans, including a prominent nose and powerful hands that could have made and used stone tools. Its brain was still relatively small, but it had long legs and an advanced hip and pelvis that would have given it more modern locomotion similar to Homo erectus and modern humans, the scientists said.

"The discovery of so many partial skeletons from a single site is unprecedented. It enables us to understand the full skeletal anatomy of these early human ancestors rather than relying on fragments of a skull or some teeth, which can sometimes be misleading," Dr Herries said.

To date, only preliminary excavations have been carried out at the site; however in the months since the Science paper was submitted more remains have been discovered along with the partial skeletons of other animals such as Dinofelis, a large-toothed cat.

Comprehensive excavations, including a study of the animal remains and isotopic analysis, are likely to reveal more information about what the climate was like and the world in which Au. sediba lived. These are scheduled for the middle of the year.

"It is likely that these fossils do not represent the oldest evidence for Australopithecus sediba. Sediments older than 2 million years occur at the site and only time will tell if they will reveal earlier examples of sediba or other species of early human ancestors," Dr Herries said.

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