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Fossils shake up our family tree

Brett Eloff / Courtesy of Lee Berger, Univ. of the Witwatersrand
Rock partially encases the cranium of a juvenile male representing the species
designated Australopithecus sediba. The skull was found in a South African cave.

Well-preserved fossils found in a South African cave pit mark an important transition between the 3.2 million-year-old pre-human known as Lucy and our own branch of the evolutionary family tree. That much, anthropologists can agree on. But exactly where do they fit in that transition? That's the subject of a high-profile debate.

It may be tempting to call these fossils a "missing link," as some of the leaked news reports did earlier this week. But scientists say that's too simplistic a term for what the bones signify.

There are so many elements to the tale that it's hard to know where to start. Here are the main points relating to the find, announced today by the journal Science:

  • Scientists say the two partial hominid skeletons are between 1.78 million and 1.95 million years old, going back to a time that has been little-documented in our ancestry.
  • The place where the bones were found, known as the Malapa cave, was identified using a Google Earth survey. The first bone was spotted not by a professional scientist, but by the 9-year-old son of one of the researchers.
  • The research team determined that the fossils came from an adult woman and a boy who might have been a relative, or even her son. Geologists surmise that they fell down the cave opening and were swept into a "death trap" that played a big role in preserving the remains.
  • The skeletons appear to represent a transitional species in the human family tree, with some characteristics in common with Lucy, and others in common with extinct members of our own Homo genus.
  • The discoverers decided to put these fossils in the same genus as Lucy, giving them the scientific name Australopithecus sediba.

"Sediba" means "wellspring" in southern Africa's Sesotho language, and the discoverers believe the species may have been the wellspring for the origins of the genus Homo. But the anthropologist who found Lucy thinks that the discoverers are wrong, and that the fossils actually represent a species within Homo - and not even the oldest example of a Homo ancestor. 

The sharp differences of opinion demonstrate that tracing human origins is not a simple exercise. The full picture is likely to be more nuanced and less sharply delineated than the neat diagrams you may have studied in biology class.

Debate over the family tree
The lead author of one of the studies in Science, Lee Berger of South Africa's University of Witwatersrand, said the fossils "fill a critical gap ... lying between the australopithecines and most probably early members of the genus Homo." He also said the fossils "might be a Rosetta Stone, effectively, to defining for the first time just what the genus Homo is."

But he shied away from using the "missing link" label.

" I don't like the use of that term, because I think it's a Victorian term," he said during a teleconference in advance of today's publication. "It implies some 'chain' of evolution." Rather, he preferred to characterize the newfound species as "highly transitional," blending characteristics from earlier australopiths and later humans.

Scientists say that australopiths like Lucy (known as Australopithecus afarensis) were capable of walking upright, but were adapted for climbing in trees as well. They had powerful upper bodies and long, strong arms suited for that purpose. They also had relatively small brains and small stature. Lucy, for example, had a brain one-third the size of ours and stood about 42 inches (1.07 meters) tall.


Science / AAAS
  The bones of a juvenile male (left) and an adult female (right), recovered from the Malapa cave pit in South Africa, are arranged on an idealized outline of an Australopithecus africanus skeleton.

The South African creatures were about 25 percent taller than Lucy, but had similar-sized skulls that Berger said might have given them a "pinhead" appearance in life. The teeth were smaller than Lucy's, like those of an early Homo species. Their legs were longer, and the pelvis was built better for striding or running on the African savanna. Those adaptations would tend to put them on the Homo side of the line.

But Berger said he and his colleagues decided to put the fossils in the Australopithecus category instead, because of the creatures' arms as well as their brain size. The arms were long, like Lucy's, and that suggested that the newfound hominids were still "dependent on the trees for some of its survival - it's clearly climbing."

Not everyone agrees with that decision. The anthropologist who discovered Lucy 35 years ago, Donald Johanson of Arizona State University, told me in an e-mail that Berger's team "missed the boat" by classifying the fossils as australopiths rather than a Homo species. In effect, Johanson was saying that Berger was putting his find on the wrong branch of humanity's family tree. (Read the full e-mail from Johanson below.)

Berger, meanwhile, said that Johanson and his colleagues might have misclassified some of the hominid fossils they found in Ethiopia. Those fossils, which date back 2.33 million years, were put in the Homo category on the basis of an upper jaw. Berger said the latest discovery suggested that a more complete set of bones was required to determine where fossils fit in the family tree.

"This may be a particularly mosaic species, you must always consider that," Berger said of his find. "You may also have to consider that we may need more, stronger criteria to include things in either genera or even possibly species." (Read Berger's FAQ about the find below.)

If the past is any guide, the debate over where these newfound fossils fit into the evolutionary family tree could go on for years. With each significant discovery, anthropologists generally find that the tree is more tangled than they previously thought. Last month's revelation about a 40,000-year-old hominid bone that didn't match up genetically with either modern Homo sapiens or extinct Homo neanderthalensis is another example of that phenomenon at work.

The debate isn't trivial: It has to do with when and where our own branch of the hominid family tree from the line of now-extinct creatures like Lucy.

How the fossils were found
The saga of how the South African fossils were found adds to the intrigue. "Google Earth is really the reason I found the site," Berger said.

The anthropologist started using the 3-D mapping program in 2008 to look for caves in a fossil-rich region outside Johannesburg known as the Cradle of Humankind World Heritage Site. He and a colleague from Australia's James Cook University, Paul Dirks, found hundreds of previously unknown caves - and that summer they went on an expedition to explore a particularly promising area.

Berger found a rich fossil site in the Malapa cave almost immediately. Two weeks later, he returned to the cave with his 9-year-old son, Matthew, and his post-doctoral student, Job Kibii. It took Matthew only a couple of minutes to find the collarbone of a hominid. When the elder Berger looked around the block of stone where the bone was found, he discovered other bones as well.

Eventually, Berger and his team uncovered the partial skeletons of two individuals that were encased in a cement-like sediment.

The bone that Matthew found apparently belonged to a juvenile male. Based on an analysis of the teeth and other bones, the team believes the child was the equivalent of 10 to 13 years old in human developmental terms - or 8 to 9 years old in actual age. The fossil will be given a nickname in a contest open to South African children, Berger said.

The researchers believe that the other skeleton is that of an adult female, perhaps in her late 20s or older. That assessment is based on the pattern of tooth wear as well as the shape of the jaw and hips. Both individuals would have been about 4-foot-2 (127 centimeters) in height. Their weights would have been around 73 pounds (33 kilograms) for the female, and 60 pounds (27 kilograms) for the male.

Several techniques, including uranium-lead dating and paleomagnetic dating, were used to determine that the bones were about 1.9 million years old. Their arrangement in the cave suggested that they died within days or hours of each other. The bones of other animals from the same epoch were found encased nearby.

Courtesy of Paul Dirks
Researcher Paul Dirks stands in the Malapa cave site shortly after it was first
discovered and before excavations began.

How the hominids died
A separate research paper in Science, with Dirks as the lead author, combines all these clues with the geological data to suggest how the hominids died.

The site might once have been a complex cave system with deep vertical shafts that might have served as "death traps" on the surface. "Animals might have been attracted to the smell of water coming from the shaft, and carnivores might have been attracted to the smell of decomposing bodies," the researchers wrote.

The two hominids might have been looking for water when they ended up among the victims of a death trap. "You may think that it's possible when you start climbing down, and then you go 'oops,' and then it turns out not to be possible, and there's only one way to go, and that's to go down," Dirks said.

Berger said the fact that the bones of many animals were found together, with no evidence of scavenging, suggests that their end came quickly. "None of them were alive enough to feed or scavenge on anything else," he said.

Later, flooding water might have washed the remains of the animals - including the hominids - deeper into the cave system, down into an underground holding area where sediment could encase the remains for preservation. The researchers suggest that the roof of the cave system eventually collapsed and became eroded, exposing the sediment so that 9-year-old Matthew could discover the spot.

Science / AAAS
A cartoon shows how two hominids might have become trapped and buried in
sediments at the bottom of a cave system.

Were the adult female and the juvenile male possibly a mother and her son? "If they behaved anything like any modern primates, including humans, they probably would have been territorial, part of the same troop," Berger said. "They would have known each other in life, and they probably would have been part of a troop. And that gives a very high probability that they would have been related to each other."

Berger said he and his colleagues are looking at ways to confirm whether there was a relationship - and suggested that they're considering molecular analysis of material extracted from the bones, as has been done with tyrannosaur tissue.

"We are seeing some organics preserved in various parts of the assemblage," Berger said. He noted that genetic analysis has never been done on hominid fossils as old as the ones found in South Africa, "but we are trying everything possible and are exploring the possibility that there could be at least proteins and possibly DNA preserved."

He also said at least two more skeletons have been found at the site, and wouldn't rule out the possibility that tools or other artifacts might be found as well. All of which means the story of the Malapa hominids is just beginning.

"They are going to be a remarkable window, a time machine of morphology into the evolutionary processes and evolutionary stresses going on at that period between 1.8 and 2 million years," Berger said.

Postscript 1: Here's a rundown of frequently asked questions (and answers) written by Berger:

What does Australopithecus sediba mean?
Australopithecus means "southern ape," after the genus of the Taung child, named by Professor Raymond Dart, also from the University of the Witwatersrand, Johannesburg. 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.

What is a hominid/hominin?
A hominid is a member of the taxonomic family that includes humans, chimpanzees, gorillas and their extinct ancestors. Hominins are members of the human branch after the human lineage split from that of chimpanzees, and thus include living humans and extinct human ancestors, such as the Australopiths. Hominins are characterised by bipedal locomotion, although this may not have been the case for the very earliest members of the group, and relatively small canine teeth. Later members of this group (those in the genus Homo) are characterized by larger brains than those of living apes like chimpanzees, bonobos, gorillas, orangutans and gibbons.

How were the fossils dated?
They were dated using a variety of methods including uranium-lead, paleomagnetic and faunal dating systems. Cosmogenic dating was used to interpret the landscape formation and to determine the depth of the cave at the time.

How were the individuals preserved?
The site where the fossils were discovered is technically the infill of a de-roofed cave that was about 50 meters underground 1.9 million years ago. The individuals appear to have fallen, along with other animals, into a deep cave, landing up on the floor for a few days or weeks. The bodies were then washed into an underground lake or pool, probably pushed there by a large rainstorm. They did not travel far, maybe a few meters, where they were solidified, as if thrown into quick-setting concrete. The rock they are preserved in is called calcified clastic sediment. Over the past 1.9 million years the land has eroded to expose the fossil-bearing sediments.

Did they die at the same time, or was it a catastrophe?
The hominin skeletons were found with the bones either in partial articulation or in close anatomical association, which suggests that both bodies were only partially decomposed at the time of deposition in the lower chamber. This further suggests that they died very close in time to each other, either at the same time, or hours, days or weeks apart.

How old is the child?
The juvenile is around 10 to 13 years old in human developmental terms. He was probably a bit younger in actual age (perhaps as young as eight or nine or so) as he is likely to have matured faster than humans. The age estimate is based on modern human standards by which the eruption stages of the teeth are evaluated and the degree of development of the growth centers of the bones.

How old is the female skeleton?
Based on the extreme wear of her teeth, she is probably at least in her late 20s or early 30s.

Did she have children?
It is likely that a female Australopith of her age would have had children.

How do you know the child is a male?
There are features of the face that help us determine that the child is a male. The muscles of the child are larger than that of the other skeleton, even though it is a child. There are also features of the pelvis that we can use to determine that it is a male.

How does this find relate to Lucy?
Australopithecus sediba is approximately a million years younger than Lucy. Some scientists feel that Lucy's species, Au. afarensis, gave rise to Au. africanus, and Lee et al. are suggesting that Au. africanus or something similar gave rise to Au. sediba.

How do you know that it is a new species?
The team compared the skeletons with all the remains of fossil hominids that have been discovered and in many ways they are absolutely unique from any fossil species found.

Why is this not the genus Homo?
The fossils have an overall body plan that is like that of other Australopiths – they have small brains, relatively small bodies and long and seemingly powerful arms. They do have some features in the skull and pelvis that are found in members of the genus Homo but not in other Australopiths. However, given the small brains and Australopith-like upper body, the team felt that keeping this species in the genus Australopithecus was the conservative thing to do.

What about Homo habilis?
Our study indicates that Australopithecus sediba may be a better ancestor of Homo erectus and it may certainly help to clear up some of this "muddle in the middle."

Why is there still rock attached to the child's skull?
Due to the fragility of the base of the cranium of the specimen and to preserve part of the adhering matrix for future research, the team has decided to leave the specimen partially in rock. The team has been able to visualize this hidden part using scanning technology.

Postscript 2: Here's the e-mail response from paleoanthropologist Donald Johanson of Arizona State University, who discovered the Lucy fossil in 1974:

"The South African finds from Malapa are most interesting, but I find it curious that the authors point to so many anatomical features that indicate that the finds belong to our genus, Homo, yet they place it in Australopithecus, so I think they missed the boat here.  Finding 1.8 million-year-old Homo in southern Africa is newsworthy since previous traces have been fragmentary and controversial.

"Additionally I do not see these fossils as evolving from Australopithecus africanus, which I believe gave rise to A. robustus in south Africa.  The specimens from Malapa are not the ancestor to later all Homo as the authors believe, since we have evidence of Homo in eastern africa at 2.33 million years. My team found an upper jaw of this age in the younger sediments at Hadar where Lucy was found, and this palate represents the oldest anatomical evidence, thus far, for our genus. It is probably best attributed to Homo habilis. Let us not forget that the Turkana Boy is about 1.8 million years old and is without doubt Homo and is attributed to Homo ergaster.

"The Malapa hominids, with so many Homo features, but with relatively short limbs, resembles Olduvai Hominid 62 which we found in the mid-'80s. Although fragmentary, OH 62 does have relatively shorter legs and longer arms, like earlier Australopithecus, and the appearance in the fossil record of a more modern body build, as in the Turkana Boy, comes later.  However the Olduvai material, OH 62 and several other specimens, are attributed to Homo on the basis of diagnostic features in the teeth, jaws and cranium.  Some scholars have suggested we place H. habilis into the genus Australopithecus and until there is a modern body build Homo should not be used as the genus for these fossils.

"It is also rather possible that Homo, like Australopithecus, underwent a diversification (adaptive radiation) resulting in several different species. This would not be unusual.  However, within the greater framework of Australopithecus and Homo, I believe emphasis should be placed on the diagnostic anatomy of the teeth, jaws and cranium…. So, I would continue to use Homo for habilis, and for these new specimens from Malapa.  Until a more comprehensive comparative study is undertaken (I know other specimens have been recovered from the site), the relationship between the Malapa material and Homo in eastern Africa is not very clear.  I would not be surprised if the Malapa material represents a newly recognized species of Homo.

"The South African finds, about half a million years younger, are probably descendants of the eastern Africa Homo. 500,000 years is a long time, and Homo could easily have migrated from eastern to southern Africa in that time.

"There are two partial skeletons, one a female and the other a male.  The skull of the male is refreshingly complete and should be attributed to Homo. Just after Lee found the first hominids at Malapa he invited me to see the material at Wits. The mandible is lightly built, not very deep or thick resembling Homo. The first and second permanent molars are erupted and there is little occlusal wear, suggesting a diet quite different from Australopithecus. In Australopithecus by the time the second molar erupts the first shows rather heavy wear.  Also, the teeth are small in size, like in Homo, and unlike Australopithecus.

"We have a very comprehensive understanding of the dating, diversity and relationships between the species of Australopithecus, but we know relatively little about the origins of our own genus. Thus, anything found that represents early Homo is potentially of some importance.  I think these finds will refocus attention on the South African fossil sites and strengthen the importance of these sites for a more complete understanding of the human family tree."

Authors of the paper about the hominid fossils include Berger, Dirks and Kibii as well as Darryl J. de Ruiter, Steven E. Churchill, Peter Schmid and Kristian J. Carlson. Authors of the paper about the geological setting include Dirks, Kibii, Churchill and Berger as well as Brian F. Kuhn, Christine Steininger, Jan D. Kramers, Robyn Pickering, Daniel L. Farber, Anne-Sophie Meriaux, Andy I.R. Herries and Geoffrey C.P. King.

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