Smithsonian.com

Although chimpanzees usually walk on all fours, sometimes they walk on two legs. New research suggests chimps walk bipedally to carry valuable resources, which might explain why bipedalism evolved in hominids. Image courtesy of Flickr user DrewLX

One of the biggest questions in human evolution is why hominids evolved upright, two-legged walking, or bipedalism. It seems to be the key trait that separated the earliest hominids from their ape cousins. New research on how wild chimpanzees walk suggests our ancestors took their first bipedal steps to free their arms and hands to carry valuable resources.

The idea that bipedalism evolved to free up the hands is not a new idea—it can be traced back to Charles Darwin. But it’s a difficult hypothesis to test with the fossil record. So a team of researchers—including Brian Richmond of the Smithsonian’s Human Origins Program—turned to chimpanzees. Many anthropologists think hominids probably evolved from an ape that was quite similar to chimps, making them good test subjects for theories related to early hominid evolution.

In the new study, published in the journal Current Biology, the researchers traveled to the Republic of Guinea in West Africa and provided piles of oil palm and coula nuts to 11 chimpanzees in a forest clearing. The chimps preferred the coula nut, which was rare in the area compared to the abundant oil palm nut. When coula nuts were provided, the chimps were four times more likely to pick up the nuts and walk away on two legs. In addition, the chimps could carry twice as many nuts while walking bipedally as when walking on all fours. The team concluded that the chimps brought the prized nuts to another location to avoid competition with other chimps—and walking bipedally was the best way to do it. To further support their findings, the team also watched crop-raiding chimps, which often ran away on two legs after stealing papayas and other cultivated plants. (You can watch a chimp in action here.)

How does this behavior relate to early hominids? If our ancestors frequently found themselves in similar situations—coming across valuable and unpredictable foods that might not be widely available—then early hominids would have benefited from collecting the precious commodities and transporting them away from the source and other hungry competitors. In turn, the team wrote, “this could reward higher frequencies and/or longer distances of bipedal bouts of carriage, creating a selection pressure for more economical bipedality.”

This is not the first time anthropologists have studied chimpanzees to gain insight on the origins of upright walking. In 2007, a team led by Herman Pontzer, now at the City University of New York, examined the energetics of captive chimpanzees walking on two legs versus four. Human walking was 75 percent less costly, as measured in oxygen consumption, than chimp walking—regardless of whether a chimp walked upright on two legs or knuckle-walked on all four, the researchers reported in Proceedings of the National Academy of Sciences. However, with only slight increases in leg length and hip extension, a knuckle-walker would save more energy if it walked upright. Such energy savings might have led to the evolution of bipedalism in hominids, the researchers suggested, as Africa became cooler and drier during the Miocene. As forests shrank, two-legged walking would have been the most efficient way to travel between isolated patches of food.

There is one sticking point with such chimp studies, however: Not all anthropologists agree that the ancestor of hominids resembled chimpanzees. In 2009, an international team of researchers published 11 papers outlining the anatomy, habitat and behavior of Ardipithecus ramidus, an early hominid that lived in East Africa 4.4 million years ago. Based on the features of the species’ hands, feet and lower back, the team concluded in Science that hominids could not have evolved from a knuckle-walker. Instead, they must have descended from an ancestor with a more monkey-like body plan. Therefore, they suggested, knuckle-walking chimps are not good models of the evolution of hominid bipedalism.

Of course, not all anthropologists agree with this interpretation of Ardipithecus. So the question of chimps’ value as models of early hominids remains open—as do questions surrounding the origins of our ancestors’ upright walking.

The skull of Sahelanthropus. What does its body look like? Image courtesy of Wikicommons

Hominid Hunting went on an unexpected hiatus in January. I’m finally back. For my first post, I thought I’d share what I’ve been thinking about for the past couple months: my fantasy fossil finds, or the hominid discoveries I’d most like to see. In no particular order:

1. The skeleton of Sahelanthropus: In 2002, anthropologists announced the discovery of a new hominid (PDF): Sahelanthropus tchadensis. Unearthed in Chad, the find was exciting because it was the first—and still only—hominid found west of Africa’s Rift Valley. And at six million to seven million years old, it was the earliest known hominid. But the species’ place in the hominid family tree is not secure. The original discovery consisted of a skull, jaw and a few isolated teeth. (Since then, researchers have found (PDF) a few additional jaws and teeth.) The position of the skull’s foramen magnum—the hole near the base of the skull where the spinal cord exits—is like that of a hominid, more forward under the skull, indicating an erect posture and upright walking. But to confirm Sahelanthropus‘ hominid status, and convince the skeptics that it’s not a non-hominid ape, scientists need to find the species’ post-cranial bones.

2. The skull of Orrorin: Around the same time that Sahelanthropus was discovered, researchers dug up another new hominid species, Orrorin tugenensis, in Kenya. Like Sahelanthropus, the hominid was very ancient, about six million years old. The discovery consisted of 13 fossils, including thigh bones, finger bones and isolated teeth and jaw fragments. The thigh bones show the telltale signs of walking upright while the rest of the known body looks more apelike, which is expected for a very early hominid. But to get a fuller picture of the species it would be nice to have a complete skull.

3. Hobbit DNA: Almost ten years after Homo floresiensis was discovered on the island of Flores in Indonesia, anthropologists still disagree about whether the hobbit was a distinct species of Homo or a diminutive modern human with a genetic growth disorder, perhaps microcephaly. Extracting DNA from one of the hobbit fossils would help resolve the debate, revealing whether or not its genetic blueprints match our own.

4. Fossils of a Denisovan: The study of the Denisovans has the opposite problem. A couple years ago, researchers discovered a potentially new hominid species based purely on its DNA. The DNA came from an isolated finger bone found in a cave in Siberia. The bone dates to between 30,000 and 48,000 years ago, a time when modern humans and Neanderthals could have lived in the area. But the genetic material didn’t match either species. So now anthropologists know there was a third type of hominid in Eurasia at this time—but they have no idea what it looked like.

5. Australopithecus skin: When researchers stumbled upon Australopithecus sediba in a South African cave, they found more than just a possible link between australopithecines and the genus Homo. Some of the 1.977-million-year-old fossils are covered in a thin layer that might be skin. If so, it would be the first time anyone has ever found fossilized soft tissue from an ancient hominid. To investigate the matter, a pair of scientists has started the open-access Malapa Soft Tissue Project to gather ideas on the best way to analyze the possible skin.

6. More Homo habilis and Homo rudolfensis fossils: Homo habilis is the earliest known member of the genus Homo, living 2.4 million to 1.4 million years ago in East and South Africa. It was given its Homo status largely because its brain was bigger than the Australopithecus brain. The species is somewhat controversial, however, with some researchers believing it really was a species of Australopithecus. The issue became even more confused when scientists decided that at least one Homo habilis fossil was different from all the others. A 1.8-million-year-old skull found in Kenya’s Lake Turkana region had a much larger brain size than any other Homo habilis—nearly 200 cubic centimeters bigger. Now some researchers place this and a few other specimens in the species Homo rudolfensis. But many questions remain. Are the two really different species or part of one variable species? Finding more of the big-brained skulls, with associated post-cranial bones, might help researchers determine how different the two forms really were.

7. The skeleton of Gigantopithecus: The largest ape that ever lived went extinct about 300,000 years ago. All researchers know about Gigantopithecus comes from a few jaws and teeth. Based on that scant evidence, some anthropologists think the ape might have stood 10 feet tall and weighed a whopping 1,200 pounds. But to more accurately determine how gargantuan the ape was, and how it moved, someone needs to find some of its post-cranial parts.

8. More Kenyanthropus fossils: In 1999, anthropologists found the skull of the 3.5-million-year-old Kenyanthropus platyops. Researchers classified the skull as a new hominid species because of its unique mix of apelike and humanlike traits. For example, the species had small earholes like a chimp’s but a much flatter face. Many anthropologists don’t agree with this classification. The skull was in bad condition when it was found, and some researcher think it is just a distorted Australopithecus afarensis skull. The only way to settle the matter is to find more skulls that look like the original, if Kenyanthropus really ever existed.

9. A chimp relative: Almost nothing is known about the evolution of chimpanzees after they split away from the human lineage. The lack of fossil evidence may be due to where chimpanzee ancestors likely lived—warm, wet forests where fossils are not often preserved. But in 2005, a pair of anthropologists reported they had found three isolated chimp teeth dated to 500,000 years ago. Whether these teeth belonged to modern chimpanzees (which would imply they are a very long-lived species) or a chimpanzee ancestor is unknown. But what’s interesting about the teeth is where they were found: the Rift Valley of Kenya. Half a million years ago this part of Africa was largely a savannah, indicating ancient chimps were not restricted to forests. Still, even with this discovery, next to nothing is known about chimp ancestry. More fossils, from an even older period, would be a great find.

10. Something unexpected: Of course, the most exciting fossil discoveries are the ones you don’t anticipate and make scientists rethink some aspect of human evolution.

This is just my personal wish list. What’s on yours?

The skeleton of Oreopithecus bambolii. Image courtesy of Wikicommons

In the 1950s, a Swiss paleontologist named Johannes Hürzeler made an intriguing discovery. In a coal mine in the Tuscany region of Italy, he unearthed dozens of fossils, including a largely complete skeleton, belonging to an ape species named Oreopithecus bambolii (the name refers to the Greek word for hill or mountain, not the delicious chocolate cookie). A jaw of the species had been found in 1872, but the new treasure trove of fossils painted an unusual picture of the ape. The ape’s features implied it walked upright on two legs, just like humans. In fact, Hürzeler thought the nine-million-old species might have been a human ancestor. Others concluded it was just an ape that had evolved human-like characteristic due to convergent evolution. Still others who saw the fossils saw no human-like traits at all.

More than 50 years later, the debate continues.

In the 1990s, researchers Meike Köhler and Salvador Moyà-Solà, both of the Miquel Crusafont Catalan Institute of Paleontology in Spain, restored and reanalyzed a collection of Oreopithecus fossils housed in a Swiss museum. They claimed features of the ape’s pelvis, spine, legs and feet resembled those of australopithecines and modern humans, new evidence that Oreopithecus was capable of walking upright and probably did so habitually. Subsequent work on hand fossils suggested the ape also had the precise gripping skills that allow humans to thread a needle or turn a key in a lock.

Despite the similarities to humans, Köhler and Moyà-Solà argued that Oreopithecus was indeed an ape and not part of our lineage. The species evolved its unusual traits because of its unusual environment. Nine million years ago, during the Miocene epoch, the world’s climate was warmer and apes lived throughout much of Europe. The region of Italy where Oreopithecus was found was a swampy island at the time. Animals on islands often evolve unusual traits. (Scientists think the hobbit, Homo floresiensis, was exceptionally small because it lived on an island.) Oreopithecus lived in a place that lacked predators, so it was safer for the ape to travel on the ground. Waking upright, rather than climbing and swinging through the trees, also saved the ape energy, the researchers suggested. But the island was far from being a Shangri-La. The confined space meant food was limited and competition was fierce. Walking upright and precise manipulative abilities may have increased the ape’s foraging efficiency.

This view of Oreopithecus was not universally accepted. Other paleoanthropologists, such as Randall Susman of Stony Brook University in New York, interpreted the fossils differently. Where Köhler and Moyà-Solà saw human traits, Susman saw typical ape characteristics, such as long arms, short legs and curved toes, features associated with tree climbing. Some studies have suggested Oreopithecus may have been similar to modern orangutans. Susman also noted the Oreopithecus fossils are poorly preserved, and some of the bones are crushed, making it difficult to draw definitive conclusions.

Researchers have yet to find additional Oreopithecus fossils, so the debate remains in a stalemate. And Oreopithecus continues to be the most enigmatic ape of the Miocene.