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Lucy, a partial Australopithecus afarensis skeleton, is one of the most famous hominid fossils ever found in Ethiopia. Image: 120/Wikicommons

Ethiopia may well deserve the title Cradle of Humankind. Some of the most famous, most iconic hominid fossils have been discovered within the country’s borders. Ethiopia can claim many “firsts” in the hominid record book, including first stone tools and the first Homo sapiens. Here’s a look at the country’s most important hominid finds.

Omo I and II (1967-1974): While excavating the Kibish Formation near the Omo River, Richard Leakey and his colleagues uncovered a partial skull and skeleton (Omo I) and a partial skull (Omo II) that are still thought to be the oldest examples of Homo sapiens. Dating to 195,000 years ago, Omo I has several features that clearly place it within our species, including a flat face, high forehead and prominent chin. Omo II, on the other hand, looks more primitive. While some researchers suggest its thicker skull and sloped forehead preclude it from being a true modern human, others say those features were probably within the range of variation for early H. sapiens.

Lucy (1974): While searching a dry gully at the site of Hadar, paleoanthropologist Don Johanson noticed a slender arm bone sticking up from the ground. He thought it belonged to a hominid. Then he noticed a thigh bone, some bits of a spine, a pelvis and some ribs. Eventually, Johanson and his colleagues unearthed approximately 40 percent of a hominid skeleton dating to roughly 3.2 million years ago. Named Lucy after the Beatles’ “Lucy in the Sky with Diamonds,” the skeleton is officially known as AL 288-1 and is arguably the most famous hominid fossil ever found. But it took a while for Johanson, with the help of paleoanthropologist Tim White, to figure out what Lucy was—Australopithecus afarensis—and her place in the human family tree. (For a firsthand account of Lucy’s discovery and the analysis of her remains, you probably can’t find a better book than Lucy: The Beginnings of Humankind by Johanson and Maitland Edey, even if some of the science is out of date.)

First Family (1975): Just a year after discovering Lucy, Johanson’s team got lucky again, finding a jumble of more than 200 A. afarensis fossils at the site of Hadar. The collection—representing as many as 17 individuals—was dubbed the “First Family” (official name: AL 333). Because the fossils contained both adults and youngsters, the First Family is a snapshot of variation within A. afarensis and offers a look at how an individual within the species might have grown up. Anthropologists are still trying to figure out what led to the demise of such a large group of hominids. A catastrophic flood is one theory; death by over-eager carnivores is another.

Australopithecus garhi (1990, 1996-1998): Paleoanthropologists Berhane Asfaw and Tim White found a partial skull and other pieces of the 2.5-million-year-old species known as A. garhi in 1990 at the site of Bouri. Since then, no additional fossils have been unearthed (or, at least, matched to the species). Not much is known about A. garhi. Based on the length of a thigh bone, the species may have had slightly longer legs, and therefore a longer stride, than Lucy’s kind. Given the species’ age and where it was found, A. garhi may have been the hominid to make the oldest known stone tools (described next).

Oldest Stone Tools (1992-1994): At 2.6 million years old, the stone choppers, or Oldowan tools, at the site of Gona are a few hundred thousand years older than any other known stone tool. But the Gona tools’ status as earliest stone tool technology was recently challenged by another Ethiopian discovery. In 2010, archaeologists claimed that roughly 3.39-million-year-old mammal bones from Hadar contained scratches that could have only been made by a stone tool, implying stone tools were an even earlier invention than scientists had thought. Other researchers remain unconvinced that the markings were made by hominid butchering. And since no actual stone tools were found along with the bones, the Gona artifacts’ title of earliest known stone tools is still safe.

Ardi (1992-1994): Older than Lucy, Ardi is the most complete skeleton of an early hominid. The first pieces of the 4.4-million-year-old Ardi were uncovered in 1992 by one of Tim White’s graduate students, Gen Suwa, in the Middle Awash Valley. White and his colleagues then spent more than 15 years digging Ardi out and analyzing the skeleton. The hominid did not look like Australopithecus, so the researchers gave it a new name: Ardipithecus ramidus. Although the species walked upright on two legs, its form of bipedalism was quite different from that of modern people or even Lucy. Its discoverers think Ardipithecus represents an early form of upright walking and reveals how apes went from living in the trees to walking on the ground.

Ardipithecus kadabba (1997): Yohannes Haile-Selassie of the Cleveland Museum of Natural History unearthed hand, foot and other bones in the Middle Awash Valley that looked a lot like those of Ar. ramidus—only the bones were almost a million years older, with an age of about 5.8 million years. Teeth found in 2002 suggested the more ancient hominids deserved their own species: Ar. kadabba. It remains one of the earliest known hominid species.

Dikika Child (2003): From the site of Dikika comes the fossil of an approximately 3-year-old A. afarensis child dating to 3.3 million years ago. Sometimes called Lucy’s baby or Selam, it’s the most complete skeleton of an early hominid child, including most of the skull, torso, arms and legs. The fossil’s discoverer, Zeresenay Alemseged, of the California Academy of Sciences, and colleagues say the fossils suggest A. afarensis grew up quickly like a chimpanzee but was beginning to evolve slower growth patterns like those of modern humans.

Herto fossils (2003): Even if the Omo I and II fossils turned out not to be members of H. sapiens, Ethiopia would still be home to the earliest known members of our species. A team led by Tim White discovered three 160,000-year-old skulls in the Middle Awash Valley. Two belonged to adult H. sapiens while the other was of a child. Due to some features not seen in modern populations of humans, White and his colleagues gave the skulls their own subspecies: H. sapiens idaltu.

Australopithecus anamensis (2006): A. anamensis, the earliest species of Australopithecus, was already known from Kenya when a team led by Tim White of the University of California, Berkeley discovered more fossils of the species further north in Ethiopia’s Middle Awash Valley. The collection of roughly 4.2-million-year-old fossils is notable because it includes the largest hominid canine tooth ever found and the earliest Australopithecus femur.

Oldowan choppers are among the oldest-known type of stone tools. Image: Didier Descouens/Wikicomons

“Becoming Human” is a series of posts that periodically examines the evolution of the major traits and behaviors that define humans, such as big brains, language, technology and art. 

For decades, anthropologists believed the ability to use tools separated modern humans from all other living things. Then scientists discovered chimpanzees use rocks to hammer open nuts and twigs to fish out termites from mounds. And then they learned tool use wasn’t even limited to apes. Monkeys, crows, sea otters and even octopuses manipulate objects to get what they want. Yet there’s no denying humans have taken technology to a completely different level. Given that our high-tech tools are one of our defining features, you’d think anthropologists would know when hominids began modifying stones to make tools and which species was the first to do so. But there’s still much to be learned about the origins of stone tools.

The oldest-known type of stone tools are stone flakes and the rock cores from which these flakes were removed. Presumably used for chopping and scraping, these tools are called Oldowan, named for Tanzania’s Olduvai Gorge, where they were first recognized. Louis Leakey first found roughly 1.8-million-year-old tools in the 1930s. But it wasn’t until the 1950s that he found hominid bones to go along with the Stone Age technology. In 1959, Leakey’s wife, Mary, discovered the species now known as Paranthropus boisei. With its giant teeth, massive jaws and relatively small brain, the hominid didn’t look very human, but the Leakeys concluded P. boisei had to be the site’s toolmaker—until the 1960s, when they found a slightly larger-brained hominid called Homo habilis (meaning “the handy man”). This more human-like hominid must have manufactured the tools, the Leakeys thought. But P. boisei and H. habilis overlapped in time (roughly 2.4/2.3 million years ago to 1.4/1.2 million years ago), so it’s been hard to definitively rule out the possibility that both types of hominids were capable of making stone tools.

It turns out neither species is probably eligible for the title of earliest toolmaker. In the 1990s, archaeologists recovered even older Oldowan tools at the Ethiopian site called Gona, dating to 2.6 million to 2.5 million years ago. Identifying the toolmaker is tricky because no fossils have been found in association with the artifacts, and there weren’t many hominid species present in East Africa during this time period to pick from. Paranthropus aethiopicus is one possibility. But so far only one skull and a few jaws of the species have been found in one area of Kenya, so not much is really known about the hominid.

A better choice might be Australopithecus garhi. The species was discovered at a site about 55 miles south of Gona, in association with animal bones that display the characteristic markings of butchering—indirect evidence of tool use. Again, not much is known about A. gahri, as scientists have only found one skull, some skull fragments and one skeleton that is tentatively considered part of the species.

Even these tools, however, are probably not the oldest stone tools, say Sileshi Semaw, director of the Gona Paleoanthropological Research Project, and the other researchers who found the Gona artifacts. The tools at this site are so well made, requiring such precision, that the anthropologists suspect that by 2.6 million years ago hominids had been making stone tools for thousands of years.

In 2010, a group of archaeologists claimed the origins of stone tools went back another 800,000 years. Shannon McPherron of the Max Planck Institute for Evolutionary Anthropology in Germany and colleagues announced they had discovered signs of butchering at another Ethiopian site, dating to 3.39 million years ago. The rib from a cow-sized hoofed mammal and the leg fragment from a goat-sized mammal contained microscopic scratches indicative of cutting and scraping to remove flesh and pounding to break open a bone to retrieve marrow. The only hominid species around at that time was Australopithecus afarensis, Lucy’s species. McPherron’s team suggested tools have not yet been found with Lucy’s kind because early tool use was probably not as extensive as it was later on. So hominids were probably making fewer tools and thus leaving behind fewer artifacts for scientists to unearth.

The case for 3.39-million-year-old stone-tool manufacturing is controversial. McPherron and colleagues acknowledge that hominids didn’t necessarily make tools to butcher their prey; they could have used naturally sharp rocks. Other researchers doubt any butchering even happened at all. Manuel Domínguez-Rodrigo of Complutense University of Madrid in Spain and colleagues say the cut marks may actually be trampling damage or scratches from the abrasive sediments the bones were buried in. Further research is needed to confirm the marks were actually made by hominids.

Although the exact timing of when hominids began making stone tools is still unsettled, at least one thing is clear: Big brains weren’t required to make simple stone tools. The evolution of bigger brains comes at least a million years after our ancestors invented the Oldowan toolkit.

Scientists disagree on whether the 2.5-million-year-old Black Skull should be called Paranthropus aethiopicus or Australopithecus aethiopicus. Image: Nrkpan/Wikicommons

Paleoanthropologists Alan Walker and Richard Leakey unearthed the Black Skull (KNM-WT 17000) in 1985 at the site of West Turkana, Kenya. The 2.5-million-year-old skull was darkened by manganese minerals in the soil where it was buried. Complete except for the crowns of its teeth, the skull appeared to match several isolated jaws and teeth previously found in East Africa. The fossils belong to the species Paranthropus aethiopicus—or Australopithecus aethiopicus, depending on who you ask. The species highlights the trouble of identifying parallel evolution, when species independently evolve similar traits, in the hominid fossil record.

The features of the Black Skull, and the related teeth and jaws, are striking. The species had massive molars and premolars, thick jaw bones and a large sagittal crest—a ridge of bone running lengthwise down the back of the skull where chewing muscles attach. All of these features align the species with the powerful masticator Paranthropus boisei, which lived in East Africa 2.3 million to 1.2 million years ago, and Paranthropus robustus, which lived in South Africa 1.8 million to 1.2 million years ago. Because of the Black Skull’s greater age, some anthropologists think it’s the ancestor of the younger P. boisei and P. robustus, and call the species Paranthropus aethiopicus. All three Paranthropus species are thought to form a dead-end side branch on the human family tree.

That’s one way to interpret the Black Skull. But other features complicate the picture.

In some ways, the Black Skull wasn’t at all like the other Paranthropus species and was instead more similar to the older, more primitive Australopithecus afarensis: It had a flat skull base, a shallow jaw joint, a protruding face and a small brain (410 cubic centimeters). In contrast, P. boisei and P. robustus had an angled skull base, a deep jaw joint, a flat face and a somewhat larger brain (500 to 545 cc)—all traits that they shared in common with early Homo. If P. boisei and P. robustus evolved from the more primitive P. aethiopicus, it means they share features with early Homo due to parallel evolution–that is, both lineages independently evolved similar cranial characteristics.

In the 1990s, Randall Skelton of the University of Montana and Henry McHenry of the University of California, Davis (one of my graduate school professors) came to a different conclusion regarding the similarities between Homo and Paranthropus. They suggested (PDF) that the two lineages actually inherited their shared features from a common ancestor, perhaps a species like South Africa’s Australopithecus africanus. In their opinion, P. aethiopicus was too primitive to be the ancestor. And in fact, the pair argued that parallel evolution, not common ancestry, explained all of the resemblances between the Black Skull and P. boisei and P. robustus; all three species must have had similar diets and therefore evolved similar chewing power. In this scenario, the Black Skull was an earlier offshoot of the Australopithecus lineage that left behind no descendants and should be called Australopithecus aethiopicus.

So, how did anthropologists come up with such different opinions about the Black Skull’s place in the human family? The answer comes down to how researchers construct their family trees, or phylogenies. The trees are made through a cladistic analysis, in which researchers, with the help of computers, group species based on the overall number of shared traits inherited through common ancestors. Different trees can arise for a number of reasons, such as how traits are interpreted and defined. For example, should large molars, thick jaws and a big sagittal crest count as three traits or one large trait complex related to chewing?

Over the years, anthropologists have constructed numerous trees that support both arguments, although the P. aethiopicus scenario appears to be the most favored as that species name is most commonly used. Regardless, the case of the Black Skull reminds us that sometimes looks can be deceiving, especially in the fossil record.

The chemistry of early Homo teeth reveals that the hominid ate more meat than Paranthropus did. Image: José Braga and Didier Descouens

Paranthropus and Homo both emerged in South Africa roughly 1.8 million years ago and lived side by side for several hundred thousand years. Differences in their diet have been used to explain why the Homo lineage succeeded while Paranthropus died out. Now, new chemical analyses of fossil teeth further confirm that the two hominids dined on different foods, with Homo eating more meat than Paranthropus. But even with these differences, the two genera appeared to have ranged over the South African landscape in similar ways.

Scientists have several methods to reconstruct the meal choices of ancient animals.

They can look at the size and shape of the teeth, jaw and skull and look at the diet of modern animals with similar characteristics. They can also observe the microscopic scratches and pits on a tooth’s chewing surface to determine how hard or abrasive one’s diet was. A third option is to investigate the chemistry of an individual’s teeth and bones, which is derived from the chemistry of what the animal ate.

Vincent Balter of Ecole Normale Supérieure de Lyon in France and colleagues selected the third method for their research, published last week in Nature. They analyzed the dental chemistry of seven Paranthropus robustus specimens, three early Homo specimens (species not known) and four members of Australopithecus africanus, which lived lived in South Africa 3.3 million to 2.1 million years ago. All of the teeth came from the famous cave sites of Sterkfontein, Swartkrans and Kromdraai.

A Paranthropus molar. Image: José Braga and Didier Descouens

Using a laser, the team removed tiny amounts of the dental enamel to measure strontium, barium and calcium isotopes. (Isotopes of an element have different numbers of neutrons.) The ratio of these isotopes tend to change as you go up the food chain. Low barium-to-calcium or strontium-to-calcium ratios, for example, are typical of carnivores. John Hawks has a good explanation of how anthropologists use such ratios to examine diet on his blog.

Looking at these isotope ratios, a clear pattern emerged. Meat was a large component of Homo‘s diet whereas plants were a big part of P. robustus‘ diet. These results are in line with previous studies. A. africanus ate both types of food. The researchers speculate the species probably ate a lot of “woody” plants (fruits and leaves, not grasses) during certain seasons and meat during other times of the year, although they can’t say which foods were eaten during which seasons. Taken together, these results suggest earlier hominids were generalists, and then around two million years ago, they began to specialize more. The addition of meat in Homo‘s diet may have allowed our ancestors to evolve big brains, which require a lot of energy to support.

The team also looked at a third isotope ratio, strontium-87 to strontium-86. Strontium isotopes vary by the geology of the local bedrock, so variations in this isotope ratio indicate hominids were eating foods in different locations. These ratios were pretty much the same for all three hominid species, suggesting they all had similar home ranges. So even though Paranthropus and Homo had different diets, they traveled around in similar areas and traversed similar amounts of territory.

To get an even better look at how diets changed with the origin of Homo and Paranthropus, Balter and his colleagues suggest similar tests should be conducted on the teeth of Australopithecus sediba—the 1.97-milion-year-old species that some anthropologists say is a candidate for the ancestor of Homo.

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.