/ 9 September 2011

Hand heritage: The telling anatomy of an ancestor

Hand Heritage: The Telling Anatomy Of An Ancestor

He may not be much of a looker but new evidence suggests Karabo, the Australopithecus sediba fossil revealed to the world last year, may have been a vital evolutionary stage in the long ascent of our species.

Scientists working on Karabo say that despite his primitive appearance, he contains a good deal of the genus Homo sapiens.

“The research will change the way we view evolution,” palaeontologist Lee Berger, a reader in evolution at Wits University’s Institute for Human Evolution, said this week. “We’d been putting scraps together. Now Karabo gives us a more complete picture to work with.”

The discovery was back in the news this week with the publication of five research papers in the authoritative periodical, Science, based not only on Karabo, a pre-teenage boy, but also on the fossil skeleton of a female in her late twenties or early thirties, unearthed at the same site.

The new theories, based on examining the foot, pelvis, hand and brain of the two creatures, tend to undermine the putative status of Homo habilis, often considered the first tool user, as the link between the australopithecines and the Homo family. Homo habilis is 200 000 to 300 000 years younger than A. sediba and, in some ways, more primitive.

The bones that Berger and his team of more than 80 scientists have been working on are viewed as transitional fossils, with a mixture of features of earlier and later species. But it is the modern, more human-like traits that have excited the scientists.

The two fossils died at about the same time. Berger’s nine-year-old son, Matthew, discovered Karabo two years ago. Entombed in an avalanche of sediment, they were almost perfectly preserved deep in the Malapa cave in the Cradle of Humankind, northwest of Johannesburg. They lived about a million years later than the famed Lucy, discovered in East Africa.

Sediba‘s exact lineage is still subject to speculation. Some scientists believe that Lucy’s species, Australopithecus afarensis, gave rise to the South African australopithecines, including Mrs Ples, the Taung Child and Little Foot, that belong to the species Australopithecus africanus.

Now researchers are suggesting that africanus — or even something earlier than africanus – gave rise to Karabo’s species.

“There is some evidence, based on primitive and advanced features of sediba‘s foot and ankle, that it does not descend from Lucy’s species or even africanus but from some as yet unidentified lineage of early hominin,” said Berger. “Additionally, the very advanced nature of sediba‘s hand suggests it may not have given rise to Homo habilis, which has a more primitive hand.

“We thought we had it all figured out, but this work raises all sorts of new questions, including the possibility of a ghost lineage.”

The papers describe a surprisingly advanced but small brain, a very evolved hand with a long thumb like that of modern humans, a very modern pelvis, and a foot and ankle shape never seen in a hominin species but which combines features of both apes and humans in one anatomical package.

“The many very advanced features found in the brain and body, and the earlier date, make it possibly a better candidate ancestor for our genus, Homo, than previous discoveries such as Homo habilis,” said Berger.

Karabo, he said, could be the direct ancestor of Homo erectus, which most palaeontologists agree is our direct evolutionary ancestor.

Kris Carlson, the brain expert on the sediba team, said the study of the brain, captured through new technology, showed a surprising mix of characteristics.

“The overall shape of [Karabo’s] brain groups closely with all humans,” he said.

The dating was also incredibly accurate, a Science paper reported.

Robyn Pickering, lead author on the paper pertaining to dating, said the new date of the fossils (1.977-million to 1.98-million years old) was one of the most accurate ever achieved in the early hominin record.

“When the fossils were announced in 2010, the age of the fossils was announced as being between 1.78-million and 1.95-million years,” Pickering said.

At that time, the team had not discovered the top of the deposit, which was only found late last year. The deposit is capped by a limestone layer that was dated using the uranium-lead dating method and found to be the same age as the flowstone at the base of the deposit.

“This allowed the team, using palaeomagnetic dating, to identify a very specific, well-dated reversal of the Earth’s magnetic poles that occurred between 1.977-million and 1.98-million years ago,” she said. The 3000 year margin of error in two million years represented an extraordinarily small 0.15%.

The team has an open access policy and sediba is already one of the most intensively studied hominin species yet discovered. The team studying these fossils is one of the largest ever assembled for research of this type and includes experts who have worked on other hominin finds.

The sediba team said they were more than willing to open up all their research and findings to anyone who was interested.

“When we made the revelations we didn’t get ripped apart, as so often happens in this small community of ours,” Berger said. “We are pushing the boundaries, but, because of our openness, there hasn’t been the usual bun fight.”

Berger said there had been very little debate about whether the fossil represented a new species. “The debate has centred, largely, on whether the species should be placed in the genus Homo,” he said.

Given sediba‘s small brain and Australopithecine-like upper limbs, as well as features of the foot and ankle, the team felt that it was wise to be conservative and to keep the species in the genus Australopithecus, Berger said.

The telling anatomy of an ancestor
Foot, ankle: walking on two legs?
Above all, the foot tells us whether a species walked upright, said the foot expert on the sediba team, Bernhard Zipfel. Karabo practised a unique form of bipedalism, the science suggests. “The ankle joint is mostly human-like in form and inferred function, and there is some evidence for a human-like arch and Achilles tendon,” explained Zipfel.

But a surprising find is that sediba‘s foot is ape-like as well, indicating that it would have climbed trees. Because parts of the heel are more primitive than that of earlier hominins such as Lucy, the implication may be that sediba did not descend from that lineage.

“No ankle has ever been described with so many primitive and advanced features in one complex, and if the bones had not been found stuck together the team could have described them as belonging to different species,” Zipfel said.

Brain: human-like but small
Karabo had a special brain, said palaeontologist Lee Berger. “If ­evolution worked the way we thought it worked, it would have looked different.”

Earlier theories put brain size ahead of organisation as a factor that determines intelligence. The idea is that our ancestors first got a big brain, which was reorganised to facilitate the higher intellectual faculties.

But Karabo’s brain, recreated using modern scan technology from the imprint on the inside of the skull, shows signs of higher organisation — in the mould of a modern a human brain. “We have bits of primitive in the brain, bits of advanced,” Berger said. “And the advanced bits are shared with the later Homo ancestors that were to come.”

Hand: Karabo the toolmaker?
Research shows that sediba has several features associated with a human-like precision grip and the ability to make stone tools.

This includes a relatively long thumb compared to the fingers — longer even than that of modern humans — that would facilitate precision grip, defined as involving one or more fingers without the active involvement of the palm.

Importantly, said team researcher Tracy Kivell, of the department of human evolution at the Max Planck Institute for Evolutionary Anthropology in Germany, Karabo had more features associated with tool-making than the “handyman” species, Homo habilis.

But, she said, sediba‘s hand also had traits suggesting that it was still capable of the powerful flexion needed for climbing trees.

“In comparison with the hand of Homo habilis, sediba makes a better candidate for an early tool-making hominin hand and the condition from which the later Homo hand evolved,” she said.

Pelvis: big, but not big enough to bear a large-brained child
Sediba‘s pelvis combines earlier hominin and later human shape and form.

But most importantly, it hits the “obstetric hypothesis” out the park.

Developed more than three decades ago, it suggests that the larger brains of early Homo babies were the reason for the human pelvis being shaped differently from that of early hominins such as Lucy, whose is broader, flatter and more flaring.

But Karabo had a small brain and a surprisingly modern pelvis, which tends to refute the hypothesis for the entire human lineage, said team researcher Job Kibii.

“It is surprising to discover such an advanced pelvis in such a small-brained creature, because of previous ideas on the origin of the shape of the human pelvis.”