Smithsonian.com

The eight bones of the new fossil foot discovered in Ethiopia. © The Cleveland Museum of Natural History

Lucy wasn’t alone. A new fossil foot unearthed in East Africa comes from an unknown hominid species that lived at the same time and in the same region as Lucy‘s species, Australopithecus afarensis. Lucy and her neighbors were both capable of walking upright on two legs, researchers say. But while Lucy spent most of her time on the ground, the newly discovered species was more adept at moving around in the trees.

“This find alters our understanding of the evolution of bipedalism because it shows that there was more diversity than previously recognized in the ways that early [hominids] moved around their environments,” says Brian Richmond, a paleoanthropologist at Smithsonian’s Human Origins Program and George Washington University.

The new discovery—eight bones from the front part of a right foot—comes from Ethiopia’s Woranso-Mille site and dates to 3.4 million years ago. This coincides with the period when Australopithecus afarensis lived in this part of Africa, about 3.0 million to 3.9 million years ago. The analysis of the bones was led by Yohannes Haile-Selassie, curator of physical anthropology at the Cleveland Museum of Natural History, and Bruce Latimer, a physical anthropologist at Case Western Reserve University in Ohio.

The researchers determined that the foot possesses features indicative of bipedal walking, such as certain joints seen in modern humans that allow the toes to push off the ground and propel the foot forward during upright walking. But the foot also appears apelike: Its opposable, grasping big toe suggests the unnamed species was a good tree climber and probably spent much less time on the ground than Lucy and later hominids, the researchers reported last week in Nature.

Haile-Selassie and his colleagues can’t give the species a name based on the scant fossil evidence. But the foot does resemble an even earlier hominid, the 4.4-million-year-old Ardipithecus ramidus, which also had an opposable big toe. Perhaps some species of Ardipithecus survived until this time.

Regardless of who the foot belonged to, it seems two types of hominids were around during this relatively early period in human evolution—and it means the evolution of bipedalism was probably more complicated than scientists suspected. For decades, the question has been what factor led the ancestors of hominids to walk upright. Now anthropologists also have to consider what factor(s) led to the origin of different styles of bipedalism.

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.

An artist’s reconstruction of Australopithecus afarensis, a species that can be called a hominid or a hominin. Image courtesy of Wikicommons

If you follow news about human evolution, you’ve probably noticed that our ancestors are increasingly called hominins rather than hominids. Why the change? It’s the result of researchers revising how they classify primates.

The system of taxonomy that biologists use to categorize animals, plants, bacteria and other organisms is based on the work of the 17th-century scientist Carl Linnaeus. It consists of nested, hierarchical groups that get more and more narrow as you go down the taxonomic chain. To understand what the terms hominins and hominids mean, let’s first look at the traditional classification of modern humans.

Kingdom: Animalia

Phylum: Chordata (animals that have a notochord at some point in their lives; in fish, reptiles, birds and mammals, the notochord becomes the vertebral column)

Class: Mammalia

Order: Primates (lemurs, bush babies, tarsiers, monkeys, apes and humans)

Family: Hominidae (modern humans and our close extinct relatives, such as Ardipithecus and Australopithecus)

Genus: Homo

Species: sapiens

Under this system, the term hominid refers to members of the Hominidae family (in taxonomy, names that end in -idae refer to a family). But in the past few decades, the definition of Hominidae has been broadened to include orangutans, gorillas and chimpanzees because of the recognition that these apes are very closely related to humans. In the past, they had their own family—Pongidae—based on the physical characteristics that seemed to unite the great apes as a group. Genetic analyses, however, indicated that gorillas and chimpanzees are actually more closely related to humans than they are to orangutans. Therefore, the Pongidae family didn’t make sense (in technical terms, it was paraphyletic). The genetic discoveries led to a new classification of humans, starting at the family level.

Family: Hominidae (orangutans, gorillas, chimpanzees and humans)

Subfamily: Homininae (gorillas, chimpanzees and humans)

Tribe: Hominini (humans and our close extinct relatives; the group that was called Hominidae in the previous classification)

Genus: Homo

Species: sapiens

Here, the term hominin refers to the tribe Hominini. That’s why many of our extinct ancestors are now called hominins. But it’s not technically wrong to call them hominids—all members of Hominini are also members of the subfamily Homininae and the family Hominidae, that’s how the nesting system works. It’s just a less precise term.

At Hominid Hunting, we generally use the term hominid in the traditional sense of the word: humans and their close extinct ancestors. But rather than being old-fashioned, I think it means we’re allowed to write about chimpanzee, gorilla or orangutan evolution from time to time.

A reconstruction of a Neanderthal, which was named after Germany’s Neander Valley. Image courtesy of Flickr user erix!

It’s hard to keep track of who’s who in the hominid family tree. It’s especially hard because the names sound so foreign—unless you happen to be well versed in ancient Greek and Latin as well as a variety of modern African languages. Here’s a quick guide to the meaning of some of the most common hominid names.

Ardipithecus: Discovered in Ethiopia in the 1990s, the genus Ardipithecus has two species: the 4.4-million-year-old Ar. ramidus and the 5.8-million-year old Ar. kadabba. Both forms were named using the local Afar language. “Ardi” means ground or floor; “ramid” means root; and “kadabba” means oldest ancestor.

Australopithecus: Raymond Dart found the first Australopithecus specimen, the Taung Child, in South Africa in 1924. To name the genus, he combined words from two classical languages: “Australis” is Latin for southern and “pithecus” is Greek for ape. Since the initial discovery, numerous species of Australopithecus have been unearthed, including Au. anamensis and Au. afarensis. In both names, “-ensis” is a Latin suffix that means originating in. The four-million-year-old Au. anamensis was first found near Lake Turkana in Kenya; in the Turkana language “anam” means lake. Like Ardipithecus, the three-million-year-old Au. afarensis was discovered in Ethiopia, with “afar” referring to the Afar region of that country.

Homo: The name Homo sapiens is Latin for “wise man.” H. habilis is the oldest known species of our genus, dated to more than two million years ago. It was originally found in 1960 in Olduvai Gorge, the site in Tanzania made famous by Louis and Mary Leakey. The name means “handy man,” referring to the Leakeys’ belief that this species made the stone tools found at Olduvai. H. erectus lived a little later than H. habilis, but was found earlier, in the 1890s on the island of Java. Its discoverer, Eugene Dubois, recognized that this ancient being walked upright, and named it Pithecanthropus erectus, or erect ape man. Later, scientists realized it belonged in our genus and changed the name. Another member of our genus that was discovered in the 19th century is H. neanderthalensis. The first Neanderthal fossil that was recognized as belonging to an ancient human was recovered from the Neander Valley of Germany. In German, “thal” means valley. (In the 20th century, the spelling of “thal” changed to “tal,” so today sometimes the “h” is dropped from the spelling of Neanderthal and the word is pronounced differently.)

Orrorin tugenensis: This species is one of the oldest hominids ever found, dating to six million years ago. In 2001, it was discovered in Kenya in the Tugen Hills region. In the local language, the name means “original man in the Tugen region.”

Paranthropus: First discovered in the 1930s by Robert Broom, this genus lived in southern and eastern Africa from about two million to one million years ago. The two species of Paranthropus—P. robustus and P. boisei—are often called robust because of their adaptations related to chewing tough foods: large cheek teeth, big cheek bones and a large crest on the top of the head where powerful chewing muscles attached. In Greek, Paranthropus means beside man. These forms have undergone numerous name changes; at one time they were considered to be species of Australopithecus. P. boisei, discovered by the Leakeys in Olduvai Gorge, was orignally named Zinjanthropus boisei: “Zinj” referred to the East African region of Zanj and “boisei” stemmed from Charles Boise, the man funding the Leakeys’ excavations.

You don’t have to go to South Africa to see Mrs. Ples, an Australopithecus africanus fossil. Image courtesy of Wikicommons

Most natural history museums don’t have human evolution exhibits, and if they do, the bones are probably reproductions. The real fossils are usually owned by and housed in the country in which they were found. Fortunately, the Internet offers several places where you can see hominid bones up close. Here are a few of my favorite sites.

Smithsonian Human Origins Program: Smithsonian’s National Museum of Natural History has a great human evolution exhibit. But for those who can’t make it to Washington, D.C., the museum scanned more than 65 fossils to create 3-D models you can play with online. With the click of a mouse, you can rotate the fossils to get a view from any angle. Each specimen includes information on when and where the fossil was found, how old it is, and in some cases, why it’s important to the study of human evolution. The museum also has online collections of artifacts and primate bones.

eLucy: Unearthed in Ethiopia in 1974, Lucy is a 40 percent complete skeleton of a female Australopithecus afarensis. At eLucy, you can compare Lucy’s bones—her legs, ankle, arms, fingers, ribs, spine, hips and jaw—with the corresponding bones of humans and chimpanzees to see what aspects of Lucy were human-like and what aspects were still primitive. The site, funded by the University of Texas at Austin, uses a lot of technical terms, but it does have a glossary and an FAQ page that provides answers to questions about how Lucy lived and basic questions about evolution. (Fun fact: Lucy’s name comes from the Beatles’ song “Lucy in the Sky with Diamonds,” which Lucy’s discoverers were listening to after they found the fossils.)

The Natural History Museum, London: Like the Smithsonian, the Natural History Museum in London has an online collection of interactive 3-D fossils. Although the collection is much smaller—it has only three skulls, of Australopithecus afarensis, Homo erectus and a Neanderthal—the site allows for side-by-side comparisons with either a modern human skull or a chimpanzee skull, or you can contrast the ancient hominids with each other.

The Middle Awash Project: The Middle Awash site in Ethiopia is home to the early hominid Ardipithecus. The Middle Awash Project maintains a database of fossils found at the site—everything from birds to hippos to monkeys to horses. There are a couple ways you can search the database, by age or by animal type. The database uses scientific names, so you may need to Google the terms if you’re unfamiliar with them. To see all of the database’s hominid fossils, choose “Hominidae” for the field called “Family” and hit search. The database has pictures of bones from Homo erectus and Ardipithecus kadabba, which lived 5.7 million years ago. Although the black-and-white pictures aren’t that pretty, you probably won’t find a similar collection of such ancient hominid bones anywhere else online.

ESRF Paleontological Mircotomographic Database: After you agree to the site’s terms and conditions, you can view images of Homo erectus, Neanderthal and early modern human fossils. Scientists created the images at the European Synchrotron Radiation Facility in France with microtomography, which uses X-rays to make 3-D images of an object. The site doesn’t provide a lot of information about the fossils—although each image includes a reference to an academic paper about the specimen—but the images are neat because they are big and detailed. The database also has pictures of invertebrate fossils such as ammonites, critters preserved in amber and ancient eel-like creatures called conodonts.

Have I missed any good sites? If you have a favorite website to view hominid fossils, let me know in the comments below.