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New Hominid Species Not A Missing Link, Scientists Say

New Hominid Species Not A Missing Link, Scientists Say

Doubt over claims that we descended from new species and disagreement about how close a cousin it was.

Thursday, April 8, 2010 - 15:30

Devin Powell, Contributor

(Inside Science) -- Two million years ago in South Africa, a juvenile ape-like creature that walked upright fell down a deep hole and died. His remains were buried until 2008, when a nine-year-old boy named Matthew literally stumbled upon them -- tripping over a log and uncovering a remarkably-preserved skull and set of fossils.

These fossils -- and the remains of a middle-aged female adult found nearby -- reveal an unusual patchwork quilt of features: some primitive and apish, others advanced and human-like, in a combination never before seen together in the same body. 

Matthew's father, Lee Berger, has raised the possibility that the earliest humans may have descended from this new species: Australopithecus sediba.

"Sediba means natural spring, fountain or wellspring in Sotho, an appropriate name for a species that might be the point from which the genus Homo arises," Berger said in statement about the find, which will be published Friday in Science.

The first hominids to walk upright 3.5 million years ago, Australopithecus, had chimp-sized brains, protruding faces, and feet built for climbing. The earliest species thought to be human (or Homo) started to emerge over 2 million years ago with bigger brains, smaller teeth and longer legs. 

Berger, a paleontologist at the University Institute for Human Evolution at the University of the Witwatersrand in Johannesburg, argues that A. sediba could have been a link between the Australopiths and the earliest Homos.

But other researchers find this "missing link" idea hard to swallow.

"This is an outstanding find and Lee should really be congratulated," said renowned paleontologist Meave Leakey, who traveled to Africa to inspect the fossils. "However I do not think that they have anything to do with Homo or the ancestry of Homo." 

The remarkably-complete remains belong to a female adult and a male child, both about 4 feet tall, that seem to have fallen into an underground cave. Their remains were carried by flowing water into deeper underground hollows and slowly buried in mud that turned to rock over the course of time. 

"Normally we have only a hip bone, a skull, a hand bone or a foot bone and often we cannot even be sure what species the fossils represent," said Leakey. "Here we have a collection of fossils from single individuals that are exceptionally well preserved and that, with further work, are likely to be even more complete."

To figure out how old these remains are, the scientists first looked at the other bones found nearby -- species of hyena and wild ass that appeared in Africa around 2.36 million years ago and had disappeared by 1.5 million years ago.

The scientists also dated the remains by calculating the amount of uranium and lead encased with them. Over time, uranium breaks down into lead at a steady rate -- a ticking clock that revealed an age of 2 million years. The rocks that formed around the bones confirm this date; they contain traces of a reversal of the Earth's magnetic field that happened around 1.9 million years ago.

This timing is problematic for the argument that the new species could be an ancestor to the human lineage. 

"We already have good diagnostic jaws of Homo older than this," said William Kimbel, director of the Institute of Human Origins at Arizona State University in Tempe. "It doesn't strike me as terribly likely that this represents a population that was the font from which all later Homo evolved."

Fossil remains show that tool-users with big brains were already walking the Earth at this time -- such as East Africa's "handy man" (Homo habilis), thought to have lived between 2.4 and 1.4 million years ago, and Homo erectus, often described as a miniature version of the modern human, the earliest fossils of which date back to 1.9 million years ago.

The immediate ancestor of these early humans has remained a mystery for scientists; few fossils have been found of hominids living just before these early humans.

Each of the four outside researchers contacted by this reporter agreed that A. sediba is an unlikely root for this branch of early humans. Instead, they consider it to be relative -- though the closeness of the relationship is up for debate.

Kimbel favors classifying sediba as a sort of sister species to early humans that shared a close ancestor. 

"I'd classify it with Homo," said Kimbel, who pointed to features of sediba that are similar to those of early humans. 

It had long legs and the shape of it hips resembled human species better adapted for walking efficiently and even running than those of more primitive species, according to Berger.

Its front teeth -- though the shaped like those of Australopithecus ¬-- are small, like those of early humans, said paleontologist Fred Spoor of University College London. 

"The tooth size reduction that is the hallmark of Homo has always been associated with the increase of animal protein in the diet," said Spoor. "Vegetable matter requires a lot of chewing and larger tooth size so as not to wear through the teeth too fast."

But Tim White of the University of California, Berkeley argued that the characteristics of the new species are more hype than Homo. He and Leakey both favor the idea that the species is more distantly related to our ancestors, an advanced Australopith that managed to survive until the time of early humans. 

White said that A. sediba is missing many of the "key characteristics" that are normally used to classify a species as human. Its brain, for example, was the size of that of the famous "Lucy" that lived hundreds of thousands of years earlier -- tiny compared to its Homo contemporaries and less than a third the brain size of that of a modern human.

He also cautioned that many of Berger's findings are based largely on the analysis of a single juvenile, whose proportions do not necessarily represent those of an adult.

"There is obviously more media bang in a spin that relates these fossils to later Homo," said White.

Everyone agrees that reaching a consensus about how close of a cousin this new species our will require more work. Further analysis awaits the newly-uncovered fossils, and the South African site promises to offer up more specimens of the new species.

 

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Devin Powell is a freelance science journalist based in Washington, D.C. His stories have appeared in Science, Science News, New Scientist, the Washington Post, Wired and many other outlets, including The Best American Science Writing 2012 anthology.