September 9, 2011
New Evolutionary Link Between Australopiths And Humans
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New analysis of two-million-year-old hominid bones found in South Africa provide the clearest evidence of evolution´s first major step toward modern humans, evidence that is leading some experts to believe the findings will change longstanding views on the origins of humans.
The well-preserved bones, from Australopithecus sediba, are from a part-human, part-ape species that have never been seen before now. The hands are similar to man, it has sophisticated ankles that helped in movement, and a surprisingly tiny but advanced brain, according to a report released Thursday.
The fossils were discovered in a sunken cave north of Johannesburg. Lead researcher Lee R. Berger of the University of Witwatersrand in South Africa, said A. sediba is the most likely candidate to be the ancestor of modern humans.
“The many very advanced features found in the brain and body, and the earlier date make it possibly the best candidate ancestor for our genus, the genus Homo, more so than previous discoveries,” Berger said in a press release.
The bones were discovered in 2008 by Berger and his then 9-year-old son, Matthew. Matthew found the first bone that was determined to belong to a child. Two weeks later Berger uncovered the fossils of a female.
Berger and colleagues examined fossils from the site and said they found the most complete hand, the most complete, undistorted hip bone and well-preserved ankle bones.
“This is giving us insight, that isn't guesswork into an area of anatomy that is crucial and critical in how human walking evolved,” Berger told Reuters reporter Jon Herskovitz in regards to the foot and ankle bones.
The hand, which had been described as a human-like at the end of an ape-like arm, had a precision grip that could have aided in making tools, research member Tracy Kivell, a researcher at Max Planck Institute in Germany, told Reuters.
The elongated thumb differs from that of apes and allows for it to grasp objects more firmly. The grapefruit-sized brain had a shape close to that of humans and may challenge theories about brain enlargement in human development, the researchers said.
The research, published in the journal Science in five segments (or papers), suggests that A. sediba was on a direct evolutionary line to Homo sapiens.
“We have examined the critical areas of anatomy that have been used consistently for identifying the uniqueness of human beings,” Berger told BBC News. “Any one of these features could have evolved separately, but it is highly unlikely that all of them would have evolved together if A. sediba was not related to our lineage.”
If the claim is proven to be correct, it would sideline nearly all other candidates in the fossil record for which similar assertions have been made.
Popular theory holds that modern humans can trace their lineage back to Homo erectus, which lived more than a million years ago. Many experts believe this animal may in turn have had its origins traced to more primitive hominids, such as Homo habilis and Homo rudolfensis.
The new evidence for A. sediba is that, although older than its “rivals,” some of its anatomy and capabilities were more advanced than these younger forms. Simply making it a more credible ancestor for H. erectus, Berger´s team claims.
The team identified the two individuals found as possibly a mother and son. They believe the two died together in some tragic accident that caused them to fall into the cave or to become trapped there. After death, their bodies were washed into a pool and became cemented in time along with the remains of several other animals.
“It's as if evolution is caught in one vital moment, a stop-action snapshot of evolution in action,” Richard Potts, director of the human origins program at the Smithsonian Institution, who was not part of the research team, told Associated Press (AP) reporter Randolph E. Schmid.
Scientists have long considered that the Australopithecus family was a primitive candidate for human ancestry. And the new research seems to further establish that belief.
Scientists used dating technologies to find the age of the sediments encasing the recently discovered fossils. Original estimates placed the age of the remains at somewhere between 1.78 and 1.95 million years old. But the new measurements narrowed the window of uncertainty to just 3,000 years, bringing the age of the remains to between 1.977 and 1.98 million years old. The refined dating measurement is important, said the team, because it puts A. sediba deep enough in time to be a realistic ancestor of H. erectus.
“This is a very interesting time in human evolution because it is when we think we should be seeing the beginnings of our genus, Homo. Previously, we've had very few fossils from this time period, so the sediba fossils are remarkable in that they are so complete,” Dr. Robyn Pickering, from the University of Melbourne, Australia, who led the dating measurements, told BBC News.
Since the discovery of the site in August 2008, 220 bones have been found of the early hominins, representing at least five individuals. The new research is based on a detailed analysis of two of the individuals.
A total of five papers based on the new evidence are to be published in the prestigious journal Science on September 9, 2011. Four of the papers each reveal new evidence on the anatomy -- including the skull, hand, foot and ankle, and pelvis -- of early hominins based on the A. sediba fossils discovered. A fifth paper covers the most accurate dates ever achieved for early hominins in Africa.
Berger said A. sediba demonstrates a surprisingly unique combination of features, never before seen in an early human ancestor. “The many very advanced features found in the brain and body, and the earlier date make it possibly the best candidate ancestor for our genus, the genus Homo, more so than previous discoveries such as Homo habilis.”
Given the open access policy of Berger´s team, A. sediba is already one of the most studied hominin species yet discovered.
The team studying these fossils is one of the largest ever assembled in the history of archaeology or paleontology. With more than 80 scientists, students and technicians from around the world, the research team consists of geologists, computer specialists, functional morphologists, anatomists, and physicists, et. al. The papers mark one of the largest researches ever published by an African-based team or university, on a single subject to appear in a journal of this magnitude.
A high-resolution X-ray scan of the male specimen´s skull has produced a virtual cast of its braincase. The casting was made at the European Synchrotron Radiation Facility in Grenoble, France. Based on the cast, the researchers estimated an adult A. sediba´s brain would have been about 173 cubic inches in volume, or about the size of a medium grapefruit.
This is smaller than much older fossils in the record, such as the famous “Lucy” specimen, A. afarensis (3.2 million years old), but, the shape of the new measurement appears to be more human-like, especially at the front. This may hint at the start of the re-organization of the brain that would be necessary to make us what we are today.
“Indeed, one of our major discoveries is that the shape and form of sediba's brain is not consistent with a model of gradual brain enlargement, which has been hypothesized previously for the transition from Australopithecus to Homo,” added Dr Kristian Carlson from the University of the Witwatersrand, one of the main authors of the paper.
Humans have a very large brain relative to their body size, nearly 4 times that of chimpanzees. Evolution from the brain of our shared ancestry with chimps has seen this radical size increase. However, the reconstructed endocast of the young male, was surprisingly small, only about 16 cubic inches larger than chimpanzees.
“The ESRF is the most powerful installation worldwide for scanning fossils, setting the standard for what can be achieved during non-destructive studies of internal structures of fossils,” noted Paul Tafforeau, staff scientist at the ESRF and a co-author of the paper.
The right-hand of the female is nearly complete. The team showed that it looks far more like a modern human hand than that from an ape. Its fingers are shorter relative to the thumb than in a chimpanzee. The hand appears to have possessed powerful muscles for grasping, suggesting A. sediba spent a lot of time in trees, moving from branch to branch. The team also argues that the hand shows dexterity capable of making simple tools.
The hand shows that major changes in the thumb usually associated with tool making “did not imply abandoning life in the trees."
"In the foot article, we're introduced to a unique and previously unknown combination of archaic and advanced traits in sediba,” explained Potts to the AP.
The fingers of A. sediba were curved, as might be seen in a creature that climbed and hung from trees. But they were also slim and the thumb was long. While the researchers feel the thumb shows capability of tool making, no tools were found at the digsite.
The fossil provides the first chance for researchers to evaluate the function of a full hand this old, said Kivell. Previously, hand bones older than Neanderthals have been isolated pieces rather than full sets. Hand bones from a single individual with a clear taxonomic affiliation are scarce in the hominin fossil record, which has hampered understanding of manipulative abilities of the hominid hand in the past.
The extraordinary manipulative skills of the human hand are viewed as a unique trait of humanity. Over the course of human evolution, the hand was freed from the constraints of locomotion and has evolved primarily for manipulation, including tool-use and eventually tool-production.
The researchers reconstructed the A. sediba hand then compared it with other hominin fossils and investigated the presence of several features that have been associated with human-like precision grip and the ability to make stone tools. They found that this early hand possessed many of these features, including the relatively long thumb compared to the fingers, longer than even that of modern humans.
The researchers said that A. sediba´s hand has more features related to tool-making than the 1.75-million-year-old “OH 7 hand” that was used to originally define the “handy man” species, H. habilis. However, A. sediba also retains morphology that suggests the hand was still capable of powerful flexion needed for climbing trees.
“Taken together, we conclude that mosaic morphology of Au. sediba had a hand still used for arboreal locomotion but was also capable of human-like precision grips,” said Kivell. “In comparison with the hand of Homo habilis, Au. sediba makes a better candidate for an early tool-making hominin hand and the condition from which the later Homo hand evolved.”
The ankle joint of A. sediba is mostly human-like in form and there is some evidence for a human-like arch and Achilles tendon. But sediba possessed an ape-like heel and lower shin bone. Though the heel bone seems primitive, the team found the front is angled. The researchers think this combination may have led to a distinctive type of walk when the creature was not climbing trees.
Whatever the correctness of the analysis, the creature did in fact have a fascinating mix of features -- some archaic, some modern.
The pelvis is short and broad like a human pelvis, creating more of a bowl shape than in earlier Australopith fossils like the famous “Lucy,” explained Job Kibii of the University of the Witwatersrand.
A popular theory has been that the modern human pelvis evolved in tandem with the gradual growth in brain volume -- facilitating the birth of babies with bigger heads. A. sediba possibly discredits this theory, said researchers, because it had a modern-looking pelvis while possessing a small brain.
That could force a re-evaluation of the process of evolution because many researchers had associated development of the pelvis with enlargement of the brain.
“It will take a lot of scrutiny of the papers and of the fossils by more and more researchers over the coming months and years, but these analyses could well be ℠game-changers´ in understanding human evolution,” Potts told the AP.
So, could this mean that A. sediba is the missing link?
Berger admits that scientists generally do not like that term, calling it “biologically unsound.”
A. sediba is a good candidate to represent the evolution of humans, he said, but the earliest definitive example of Homo is 150,000 to 200,000 years younger.
Scientists usually prefer the terms “transition form” or “intermediary form,” Darryl J. DeRuiter of Texas A&M University told the AP.
“This is what evolutionary theory would predict, this mixture of Australopithecine and Homo,” he said. “It's strong confirmation of evolutionary theory.”
But it´s not yet an example of the genus Homo, DeRuiter said, though it could have led to several early human forms, including Homo habilis, Homo rudolfensis or Homo erectus – all considered early distant cousins to man, Homo sapiens.
“It's a great find,” he adds, “because it provides strong confirmation for Darwin's theories about evolution.”
This research forces “a rethinking of how traits are coupled together in human evolution,” Potts said in an email to AP's Schmid.
“This isn't the end of the story. What may be happening is that there were several australopithecine forms all evolving human-like features in parallel as they turned to meat-eating and tool-making and moving greater distances,” Stringer told BBC News.
“The question now is to pull out of this mess which one is really the ancestor of the genus Homo. We know there are more remains to come from this incredible site. Let's see if other individuals also show this mix of features,” he concluded.
After the bones were discovered, the children of South Africa were invited by researchers to name the child, which they called “Karabo,” meaning ℠answer´ in the local Tswana language. The older specimen has not yet been given a nickname, said Berger.
The juvenile would have been aged 10 to 13 in terms of human development. The female was in her 20s and there are indications that she may have given birth one time. Researchers are not sure if the two were related.
Image 1: The hand bones of an adult female Au. sediba as they were found in the matrix at Malapa, South Africa: An almost complete right hand was found clustered together with the rest of the upper limb. © Peter Schmid
Image 2: All of the right hand bones rearticulated. © Peter Schmid
Image 3: A 3-D rendering of the skull of Australopithecus sediba from an experiment at the ESRF beamline ID19. Credit ESRF/P. Tafforeau.
On the Net:
- University of Witwatersrand
- Max Planck Institute
- Science Special Feature
- European Synchrotron Radiation Facility
- Texas A&M University