View Full Version : Excavation of Human Ancestor Fossils Begins in South African Cave

11-06-2013, 01:45 PM
Excavation of Human Ancestor Fossils Begins in South African Cave

Lee Berger leads a team that will unearth newly discovered fossils.

Dr. Lee Berger excavates a finger bone in South Africa in 1995.

A harrowing expedition into the tiniest recesses of a cave system begins today in South Africa. The effort aims to recover recently discovered fossils of a yet-to-be-identified species of early human.

Over the next several weeks, the expert team, directed by National Geographic Explorer-in-Residence Lee Berger of South Africa's University of Witwatersrand, will delve into the Rising Star cave system outside Johannesburg to carefully retrieve the fossils.

Berger's team made headlines in 2010 with the announcement of the discovery of Australopithecus sediba, a 1.78-million-year-old species related to early human species of the era. Those fossil finds were made at South Africa's Malapa Cave site. (See "Malapa Fossils.")

The team that made the latest discovery is made up of experienced caver-scientists from around the world. All have passed Berger's requirement of being small enough to fit into and out of cramped cave passages while being able to work effectively inside the Rising Star site.

Without knowing exactly which species the bones come from, what's driving the team to face the challenges required to recover them? The answer is all in the questions.

What Did They Look Like?

When talking about extinct species related to modern humans and found in South Africa, there are two main groups that are fairly well known popularly and scientifically.

The group Homo gets its name from the Latin for "same" (as in "homophones"). These are our closest relatives, and all would likely have been pretty recognizable as such.

Species such as Neanderthals and Homo erectus are known from many bones discovered at sites across Africa, Asia, and Europe, so we know a good deal about their entire bodies, male and female, old and young.

Further back, say three million years ago or more, there is Australopithecus, from Latin for "southern" (as in "Australia") and Greek for "ape" (as in ... ok, that one has no other English connection).

The faces and arm lengths of these creatures would appear more generally apelike to our eyes. "Lucy" is a famous example from East Africa and one of the most complete early hominin skeletons yet found. Her species is called Australopithecus afarensis.

Other finds have been more modest: just a few recognizable teeth in a section of lower jaw.

A third group, between those two chronologically, is Paranthropus, which combines "para" meaning "next to" or "not quite" (as in "paranormal") and "anthro" meaning "man" (as in "anthropology" or "android").

Paranthropus were similar to the australopithecines but had bigger, flatter teeth and stronger jaws, which have tended to be the best preserved pieces.

Every new fossil, especially when found at a new location, could add pieces to any of the puzzles surrounding these species, and help flesh out the full creature behind the initial partial discoveries.

How Are We All Related?

Given these and other groups of hominids running around some one to three million years ago, how do we know which ones are actually ancestral to us?

Researchers have spent generations drawing and redrawing a family tree that could connect all the known hominin fossils. Each new find has the possibility of adding clues (or muddying the water).

As mentioned, Berger's best-known discovery was the nearly complete skeleton of Australopithecus sediba. Though nearly two million years old, it had some features more humanlike than even those of Homo habilis, who lived only one million years ago.

At the same time, A. sediba showed other similarities to much more primitive, tree-dwelling primates. (See "Human Ancestor May Put Twist in Origin Story.")

It was only the luck of finding the complete skeleton of a previously unknown species that made anyone consider such combinations of traits.

"[With A. sediba,] you have a heel bone and an ankle bone that if you found them independently, you'd put them in different taxa," said paleontologist Bernard Wood of George Washington University in Washington, D.C.

So does that make it more or less likely to be our ancestor?

"These sorts of issues really need to be worked out by finding a suite of fossils that have some time depth to them," said Rick Potts, director of the Smithsonian's Human Origins Program.

Then "you can begin to understand what sort of combination of traits would occur in a small isolated group," or that would be found in "a lineage with integrity to it [that] you see over and over again for generations."

These issues are present in a lesser degree for almost all hominin fossils. Each have unique combinations of traits that make them seem so close and yet so far from being a definite ancestor for us. This brings up the next big question.

Are We All Hybrids?

The story of human evolution was once pitched as a search for the "missing link," an ancient creature that would exhibit traits somewhat like a human, but somewhat like the other great apes. (Also see "New Studies Shake Up Human Tree.")

After a century of discoveries, it's clear there wasn't just a missing link, there was a whole missing spiderweb of connections.

Hundreds of fossils from sites all across Africa representing a dozen species or more—all with different combinations of apelike and humanlike features—have shown that the roots of our family tree are as convoluted and difficult to follow as any royal line's succession.

"There are multiple places within sub-Saharan Africa that were probably undergoing quite a complex series of responses to environmental variability," said Potts. It was not just gradual climate change, but rather the kind of instability that is now seen as playing a critical role in driving adaptation and speciation (the formation of new species).

The environmental instability of the time "creates a much more experimental world," Potts added. "Not a 'cradle of humankind'—that's too nurturing. I now prefer the term 'cauldron of humankind.'

"And it's much more of a series of broiling events and a churning process full of this sort of experimentation."

As the landscape changed, diversifying populations could meet back up and essentially jump back into the gene pool together.

Potts doesn't suggest hybridization explains all the mixed characteristics in fossils like A. sediba, but says it's a piece of the puzzle that shouldn't be forgotten when trying to trace which species absolutely did or did not contribute to the human family line.

What Don't We Know?

The final driver for continued pursuit of early hominin fossils is the growing awareness of just how much we don't know, scientists say.

A. sediba was not only an unknown species until a few years ago, but the place it was discovered also was considered exhausted of all fossils. One of Berger's main points is that our age of discovery is nowhere near done. We haven't even finished looking at the things we thought we knew, he says.

There's an overarching mystery to the whole process of interpretation of fossils. GWU's Wood thinks a lot of the confusion over determining relationships based on the shapes of the bones could be self-inflicted.

"It's not just a question of finding more fossils," he said. "We need a better understanding of how reliable morphology is for telling us about relationships."

Follow the New Expedition

The latest chapter in the search for answers is about to kick off in the Rising Star caves, but for now the emphasis is wholly on the search.

"Our aim is to get the fossils out carefully, study them, compare them to other fossil material from around the world, and then proceed to analyze and describe them," said Berger. He hopes to publish the team's findings in late 2014. A National Geographic/NOVA documentary will tell the full story of discovery and analysis next fall.

Source: http://news.nationalgeographic.com/news/2013/11/131106-lee-berger-human-ancestor-fossil-excavation/