Smithsonian.com

A trio of upright walkers: Lucy (middle) and Australopithecus sediba (left and right). Image: Compiled by Peter Schmid courtesy of Lee R. Berger, University of the Witwatersrand/Wikicommons

Welcome to Hominid Hunting’s new series “Becoming Human,” which will periodically examine the evolution of the major traits and behaviors that define humans, such as big brains, language, technology and art. Today, we look at the most fundamental human characteristic: walking upright. 

Walking upright on two legs is the trait that defines the hominid lineage: Bipedalism separated the first hominids from the rest of the four-legged apes. It took a while for anthropologists to realize this. At the turn of the 20th century, scientists thought that big brains made hominids unique. This was a reasonable conclusion since the only known hominid fossils were of brainy species–Neanderthals and Homo erectus.

That thinking began to change in the 1920s when anatomist Raymond Dart discovered the skull known as the Taung Child in South Africa. Taung Child had a small brain, and many researchers thought the approximately three-million-year-old Taung was merely an ape. But one feature stood out as being human-like. The foramen magnum, the hole through which the spinal cord leaves the head, was positioned further forward under the skull than an ape’s, indicating that Taung held its head erect and therefore likely walked upright. In the 1930s and 1940s, further fossil discoveries of bipedal apes that predated Neanderthals and H. erectus (collectively called australopithecines) helped convince anthropologists that walking upright came before big brains in the evolution of humans. This was demonstrated most impressively in 1974 with the finding of Lucy, a nearly complete australopithecine skeleton. Although Lucy was small, she had the anatomy of a biped, including a broad pelvis and thigh bones that angled in toward the knees, which brings the feet in line with the body’s center of gravity and creates stability while walking.

In more recent decades, anthropologists have determined that bipedalism has very ancient roots. In 2001, a group of French paleoanthropologists unearthed the seven-million-year-old Sahelanthropus tchadensis in Chad. Known only from a skull and teeth, Sahelanthropus‘ status as an upright walker is based solely on the placement of its foramen magnum, and many anthropologists remain skeptical about the species’ form of locomotion. In 2000, paleoanthropologists working in Kenya found the teeth and two thigh bones of the six-million-year-old Orrorin tugenensis. The shape of the thigh bones confirms Orrorin was bipedal. The earliest hominid with the most extensive evidence for bipedalism is the 4.4-million-year-old Ardipithecus ramidus. In 2009, researchers announced the results of more than 15 years of analysis of the species and introduced the world to a nearly complete skeleton called Ardi.

Although the earliest hominids were capable of upright walking, they probably didn’t get around exactly as we do today. They retained primitive features—such as long, curved fingers and toes as well as longer arms and shorter legs—that indicate they spent time in trees. It’s not until the emergence of H. erectus 1.89 million years ago that hominids grew tall, evolved long legs and became completely terrestrial creatures.

While the timeline of the evolution of upright walking is well understood, why hominids took their first bipedal steps is not. In 1871, Charles Darwin offered an explanation in his book The Descent of Man: Hominids needed to walk on two legs to free up their hands. He wrote that “…the hands and arms could hardly have become perfect enough to have manufactured weapons, or to have hurled stones and spears with a true aim, as long as they were habitually used for locomotion.” One problem with this idea is that the earliest stone tools don’t show up in the archaeological record until roughly 2.5 million years ago, about 4.5 million years after bipedalism’s origin.

But after the unveiling of Ardi in 2009, anthropologist C. Owen Lovejoy of Kent State University revived Darwin’s explanation by tying bipedalism to the origin of monogamy. I wrote about Lovejoy’s hypothesis for EARTH magazine in 2010. Lovejoy begins by noting that Ardi’s discoverers say the species lived in a forest. As climatic changes made African forests more seasonal and variable environments, it would have become harder and more time-consuming for individuals to find food. This would have been especially difficult for females raising offspring. At this point, Lovejoy suggests, a mutually beneficial arrangement evolved: Males gathered food for females and their young and in return females mated exclusively with their providers. To be successful providers, males needed their arms and hands free to carry food, and thus bipedalism evolved. This scenario, as with all bipedalism hypotheses, is really hard to test. But earlier this year, researchers offered some support when they found that chimpanzees tend to walk bipedally when carrying rare or valuable foods.

Another theory considers the efficiency of upright walking. In the 1980s, Peter Rodman and Henry McHenry, both at the University of California, Davis, suggested that hominids evolved to walk upright in response to climate change. As forests shrank, hominid ancestors found themselves descending from the trees to walk across stretches of grassland that separated forest patches. The most energetically efficient way to walk on the ground was bipedally, Rodman and McHenry argued. (Full disclosure: Rodman was my graduate school advisor.) In 2007, researchers studying chimpanzees on treadmills determined that the chimps required 75 percent more energy while walking than two-legged humans, providing some evidence that bipedalism has advantages.

Numerous other explanations for bipedalism have been outright rejected, such as the idea that our ancestors needed to stand up to see over tall grass or to minimize the amount of the body exposed to the sun in a treeless savannah. Both ideas were debunked by the fact that the first hominids lived in at least partially wooded habitats.

Although difficult to study, the question of why bipedalism evolved might come closer to an answer if paleoanthropologists dig up more fossils of the earliest hominids that lived seven million to six million years ago. Who knows how many species of bipedal apes they’ll find. But each new discovery has the potential to fundamentally change how we understand the origins of one of our most distinctive traits.

The seven-million-year-old Sahelanthropus tchadensis skull, known as Toumai, viewed from different sides. Image: Didier Descouens/Wikicommons

Ten years ago, an international group of anthropologists made a bold claim: They had unearthed the earliest hominid ever found, in the Sahel region of Chad. They named their discovery Sahelanthropus tchadensis. Today, many anthropologists agree that the seven-million-year-old Sahelanthropus was an early hominid while others suggest it was nothing more than an ancient ape.

The team, led by Michel Brunet, now at the Collège de France, originally found six hominid specimens in Djurab Desert of northern Chad in 2001. The discovery included a nearly complete, yet distorted, skull (nicknamed Toumaï, meaning “hope of life” in the local Goran language). Although very primitive, the skull, jaw and teeth displayed some hominid-like traits. For instance, the species had a relatively flat face instead of a protruding muzzle like a chimp. And the tip of the canine tooth was worn down, as it is in humans. This suggested Sahelanthropus lacked a “honing” complex in which the back side of the upper canine sharpens itself against the lower first premolar (what your dentist might call a bicuspid). This appears to be a trait that hominids lost after they split from the chimpanzee lineage. In addition, Sahelanthropus’ foramen magnum—the hole at the base of the skull that the spinal cord runs through—was situated further forward than a chimp’s, implying Sahelanthropus had an erect posture and therefore walked upright on two legs. In 2005, the team announced additional jaw and teeth discoveries from Djurab, as well as a virtual reconstruction of the skull that corrected the distortion. These new pieces of evidence supported the original find, the researchers said.

A reconstruction of Sahelanthropus tchadensis. Image: dctim1/Flickr

Based on the type and ages of other animal fossils found near Sahelanthropus—including freshwater fish, crocodiles, rodents and monkeys—the researchers concluded the species probably lived in a wooded environment near a lake, perhaps even in a swampy locale, six million to seven million years ago. Assuming the species was indeed a hominid, the time period implies the hominid-chimpanzee split must have occurred even earlier, contrary to some genetic studies that indicate a more recent split some five million years ago. And finding the hominid in Chad means early hominids lived beyond East Africa and were more spread out than paleoanthropologists had suspected.

But Sahelanthropus‘ hominid status is not universally accepted. In 2006, one group of researchers, including Milford Wolpoff of the University of Michigan and John Hawks of the University of Wisconsin, considered the structure and function of the reconstructed Sahelanthropus skull. Although the placement of the foramen magnum appeared similar to humans’, other aspects of the skull would have prevented the species from keeping its head upright—and therefore it couldn’t have been a bipedal walker, the team concluded. Thus, they suggested, Sahelanthropus was not a hominid, just some kind of ape. They further noted that some of the dental similarities Sahelanthropus shared with hominids could be cases of parallel evolution, when closely related species independently evolve similar traits due to shared evolutionary pressures.

Since 2006, the study of Sahelanthropus hasn’t advanced all that much. No additional fossils have been discovered—or at least, none of have been publicly announced. In 2009, Hawks blogged about the possibility of a Sahelanthropus femur. One of the researchers involved in the discovery of the species published a paper alluding to a thigh bone and even published a picture allegedly showing the original cache of fossils that included a femur.

As far as I know, a formal analysis of the bone has never been published. If there is a Sahelanthropus, studying it might help confirm whether the species walked upright—and whether it deserves to be included in the hominid family. Sometimes it takes scientists a long time to fully analyze a fossil find. It took the team that found Ardi and other Ardipithecus fossils about 15 years to publish full studies on that early hominid. So maybe in another five years Brunet and his team will have another announcement to make.

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?