Ancient humanlike skeleton is 'weird mosaic'

Ancient humanlike skeleton is 'weird mosaic'
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POSTED: Sunday, April 14, 2013 - 12:00am

UPDATED: Sunday, April 14, 2013 - 12:04am

If you could time travel to 2 million years ago in South Africa, you might see a creature with humanlike hands and an ape-sized brain, walking upright with feet twisted inward.

Would you recognize this as your relative?

Anthropologists are keen on exploring the mysteries of human evolution presented by the fossilized remains of a species called Australopithecus sediba, or A. sediba for short. The latest collection of studies, published Thursday in the journal Science, presents more detail than ever about what this creature was like. Whether it's a direct ancestor of humans is controversial, however.

"We continue to just see this weird mosaic of features," said Duke University anthropologist Steven Churchill, lead author of a study on the upper limb of A. sediba.

It's possible that around this time, 2 million to 3 million years ago, many different forms of upright creatures emerged in parallel, representing variations that evolved in response to environmental changes.

The A. sediba remains were discovered in South Africa in 2008. Lee Berger, paleoanthropologist at the University of the Witwatersrand in Johannesburg, South Africa, led the discovery of the fossils. Researchers announced the discovery of the fossils in 2010. "Australopithecus" means "southern ape," and "sediba" means "natural spring" or "fountain" in the Sotho language.

A. sediba individuals were about the size of chimpanzees. They had small brains and long arms that were good for climbing and hanging from trees -- which are all primitive traits. In other respects, they were humanlike, with their small teeth and chewing muscles, not to mention hands adapted for tool use.

And then there are ways in which this creature was like nothing else we know. The analysis of A. sediba's leg and foot, led by anthropologist Jeremy DeSilva of Boston University, suggests a unique example of the biomechanics of walking.

"To me, it opens up this wonderful world of possibilities of all these different ways that early humans were walking, all these different experiments in bipedalism," DeSilva said.

The completeness of the skeletons is "just extraordinary," he said. He noted that the female skeleton has a heel, ankle, hip, knee and lower back -- the five components necessary for knowing how a creature walked.

Lucy, the famous 3.2 million-year-old remains of the species Australopithecus afarensis discovered in East Africa in 1974, is not as complete, DeSilva said. Her foot and lower limb structure is also different from that of A. sediba, suggesting a different way of walking, he said.

The A. sediba heel is relatively small, like a chimpanzee's, DeSilva said. Lucy, on the other hand, had a larger heel like those of modern humans. It's surprising that A. sediba, living 1 million years later, would have a more primitive heel. Researchers have examples of the small heel from two individuals, so it's less likely this was an abnormal phenomenon.

Unlike a chimpanzee, which walks crouched over, DeSilva said, A. sediba would have had a straight leg and a fully extended hip while walking. But unlike modern humans, there was no heel strike when A. sediba walked.

"When you hit the ground, there's a tremendous amount of force. That needs to be distributed over a large area, otherwise you have increased stress," DeSilva said, explaining why humans evolved with larger heels.

A. sediba's tiny heel suggests that, when it walked, it must have been landing in a flat-footed manner but on the outside of the foot.

"The whole foot itself looks to be twisted inwards," he said. "By landing on the outside of this twisted-in foot, the ground is going to push back with considerable force and roll that foot to the inside."

There are humans today with feet twisted inward, although no one has a tiny heel like that of A. sediba, he said. In modern humans, walking this way leads to problems with the hips, knees and lower back.

Boston University physical therapist and study co-author Kenneth Holt helped DeSilva's group figure out how A. sediba's anatomy related to this strange way of walking, predicting some features before he even saw them, DeSilva said.

A. sediba has several other features that make it humanlike: The chewing muscles are relatively small, for example, and its pelvis is somewhat basin-shaped. It also appears to have an Achilles tendon, Churchill said, and there are moderate indicators that its nose stuck protruded from it face the way human snouts do.

The brain of A. sediba is also curious. The asymmetry of its shape is similar to modern humans, but not to other Australopithecus species members. Researchers reported in 2011 that the skull has a cranial capacity of 420 cubic centimeters (26 cubic inches), whereas a chimpanzee's is about 380 cubic centimeters. Homo erectus, which is believed to be a direct ancestor of humans, had a cranial capacity of 900 cubic centimeters.

Here lies another curious element to the discovery: A. sediba is only about 200,000 years older than Homo erectus, so if it is the ancestor of the Homo genus, its brain size must have expanded rapidly in evolution.

So is A. sediba a direct ancestor of our genus, Homo (we are Homo sapiens), or did it evolve in parallel?

Churchill argues that Australopithecus sediba is an ancestor of, or at least a sister species to, the direct lineage of humans. But paleontologists who believe this are in the minority, he said.

DeSilva recognizes all of the humanlike qualities of A. sediba that support this notion, but he is not sure exactly how modern humans are related to A. sediba.

"It tells a very conflicting story," he said of the research.

For a stronger argument, archaeologists would have to find fossilized remains of creatures that come before and after A. sediba in the evolutionary lineage, Berger told CNN in 2011.

Ian Tattersall, paleoanthropologist and curator at the American Museum of Natural History in New York, wasn't involved in the studies, but he told CNN in 2011 that it's unlikely we will discover a direct evolutionary path, over millions of years, from Australopith into Homo. Instead, it appears there were many forms of upright creatures -- natural experiments in evolution -- that coexisted at the same time.

The new analyses are based on remains from three A. sediba skeletons -- a young male, an adult female, and an isolated shinbone from another adult.

These remains came from a cave at the Malapa site in South Africa that lost its roof. Lime miners, about 100 years ago, blasted out big chunks of sediment from the area that contained these fossils. They were preserved in blocks of mud and rock that had set like concrete, Churchill said.

Researchers have not dug in the cave itself yet, but they have already seen more bones sticking out of the cave wall, Churchill said. Experts have recovered, from the same place, exoskeletons of insects, plant material and porcupine quills that still contain black pigment -- all from around the same time as A. sediba. Blocks of concrete-like material that have been recovered, but not studied, also contain more pieces of skeletons.

"We're going to keep recovering more and more of these hominins," Churchill said, referring to early human relatives. "I'm sure there will be more surprises yet to come." 

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