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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.

The large teeth and massive jaw of Paranthropus boisei suggest the hominid ate hard objects, but the chemistry and wear on the teeth indicate the species consumed grasses or sedges. Image courtesy of Wikicommons

It’s not hard to understand why Paranthropus boisei is often called the Nutcracker Man. The hominid’s massive molars and enormous jaw make it seem pretty obvious that the species spent a lot of time chomping on hard nuts and seeds. Yet, the only direct evidence of P. boisei‘s meals—the chemistry and microscopic scratches of the teeth—hint that the species probably didn’t crack nuts all that much, instead preferring the taste of grass. A team of anthropologists that recently  reviewed the possible diets of several early hominid species has highlighted this paradox of the Nutcracker Man and the difficulties in reconstructing the diets of our ancient kin.

The first place anthropologists start when analyzing diet is the size and shape of the hominid’s teeth and jaws. Then they look for modern primates that have similar-looking dentition to see what they eat. For example, monkeys that eat a lot of leaves have molars with sharp cusps for shearing the tough foliage. On the other hand, monkeys that eat a lot of fruit have low, rounded molar cusps. If you found a hominid with either of those traits, you’d have a starting point for what the species ate.

But the morphology of a species’ teeth and jaws only shows what the hominid was capable of eating, not necessarily what it typically ate. In some cases, these physical traits might reflect the fallback foods that a species relied on when its preferred foods were unavailable during certain times of the year. Frederick Grine of Stony Brook University in New York and colleagues point this out in their recent review in the American Journal of Physical Anthropology.

Grine and colleagues note that other lines of evidence directly record what an individual ate. One method is to look at the chemistry of a tooth’s dental enamel. As the enamel forms, atoms that an individual consumes become incorporated in the tooth. One of the most common elements to look for is carbon. Because different plants have unique ratios of carbon isotopes based on how they undergo photosynthesis, the carbon isotopes act as a stamp that records what the individual once ate. Researchers look for two main plant groups: C3 plants are trees, fruits and herbaceous plants that grow in environments with cooler seasons while C4 plants are the grasses and sedges that grow in tropical, warm regions. Finding the isotopic traces of C3 or C4 plants in teeth indicate a hominid ate those plants (or animals that ate those plants).

Another way to directly sample diet is to look at the characteristic microscopic markings on a tooth’s surface that form when chewing certain foods. Eating tough grasses and tubers, for example, will leave behind scratches; hard nuts and seeds create pits. One drawback of this method is that a tooth’s microwear is constantly reshaped whenever an individual eats. So, the markings found by anthropologists probably represent an individual’s “last meal,” whatever he or she was eating in the days before death. If a hominid had a diet that changed seasonally, part of the diet may not be reflected in the tooth’s surface wear.

With all of these methods in mind, Grine and his colleagues considered the probable diets of several early hominid species. A comparison of the closely related P. bosei and Paranthropus robustus emphasized the puzzle of the Nutcracker Man.

P. robustus lived in South Africa 1.2 million to 1.8 million years ago when the region was an open grassland. The species’ giant, thickly enameled molars and premolars (better known as bicuspids) and heavy jaw suggest P. robustus was chewing hard objects. The surface wear on the teeth also point to eating hard foods and resemble the wear patterns seen in modern mangabey monkeys, which often eat nuts. The teeth’s enamel chemistry further supports this conclusion: As much as 60 percent of the species’ diet consisted of C3 plants, which would include hard-shelled nuts and fruits (carbon chemistry can’t detect which part of a plant an animal ate).

P. boisei lived in the wooded and open grasslands of East Africa at about the same time P. robustus was alive. It had an even larger jaw and teeth, with the biggest molars of any hominid. These traits indicate the species was a powerful chewer. But the wear patterns on the molar lack the deep pits that characterize those of hard-object eaters. Instead, the patterns match those of gelada baboons, which eat a lot of tough grasses. A grass diet is further hinted at by the carbon isotopes in P. boisei teeth: As much as 77 percent of their diet consisted of C4 plants (grasses and sedges).

Grine and his colleagues suggest there may be a way to reconcile the paradox of P. boisei. Instead of being adaptations to cracking open hard objects, the species’ massive teeth and jaws may have been traits that helped P. boisei handle very abrasive foods, including any grit clinging to blades of grass. Or perhaps the species’ used its giant molars to grind its food in a unique way. These are ideas that anthropologists should further investigate.

Although  P. boisei‘s diet seems puzzling, one thing is clear: The apparent mismatch between the various lines of evidence demonstrate that anthropologists still have a lot to learn about what our ancestors ate.

Mary and Louis Leakey. Image courtesy of the Smithsonian Institution Archives

Louis Leakey was not the first person to ever find an ancient hominid fossil. But more than anyone else, he promoted and popularized the study of human evolution. His work spurred others to go to Africa to find our ancestors’ remains, he and his wife raised their son to go into the family business, and he initiated some of the first field studies of our closest living relatives, the great apes, as a way to understand early hominids. For all of these accomplishments, I call Leakey the Father of Hominid Hunting.

Leakey was born and raised in Kenya. He found is first stone tools as a teenager, which helped convince him that Africa was the homeland of humankind. That put him in the minority. During the first half of the 20th century, anthropologists considered Asia, or perhaps Europe, to be the birthplace of humans. That’s where all of the hominid fossils had been found.

That didn’t deter Leakey. In 1926, he set off for his first archaeological expedition in East Africa. It was just one year after Raymond Dart announced the discovery of the Taung Child, an australopithecine and the first hominid fossil to be recognized in Africa. His goal was to find the earliest fossil of our genus, Homo. But for the next three decades Leakey’s expeditions uncovered only stone tools and the first fossil skull of the earliest known ape, the 18-million-year-old Proconsul. It wasn’t until July 1959 that Leakey’s wife, Mary, while working in Tanzania’s Olduvai Gorge, found a hominid bone.

It was a skull, but not exactly the skull Leakey’s team had been looking for. Based on the skull’s giant teeth and small brain, it was clear that the hominid was not a member of Homo. But Leakey and his wife were excited about the find anyway. They named it Zinjanthropus boisei (now known as Paranthropus boisei) and declared “Zinj” had made the stone tools found nearby (that’s still a matter of debate). Leakey asked Phillip Tobias, a South African anthropologist who died last week, to analyze the skull. Tobias determined it was an australopithecine; the fossil especially resembled Australopithecus (now Paranthropus) robustus, first found in South Africa in the 1930s. Zinj, eventually dated to 1.75 million years ago, was the first australopithecine found outside South Africa.

Even though Mary actually found the fossil, Leakey received much of the credit and became a celebrity—traveling around the world to talk up the discovery and drum up financial support for their fieldwork.

More success came in the early 1960s. Mary found additional fossils at Olduvai. But they were different from Zinj. With somewhat larger brains, the fossils looked more human, Leakey thought. He decided the remains represented the earliest member of our genus and our direct ancestor. He called the species Homo habilis, or “handy man.” It was the discovery Leakey had been spent his career looking for.

To this day, H. habilis remains one of the most controversial species in the hominid family. Paleoanthropologists disagree on whether the fossils represent one or more species—and whether they’re even Homo or not. Perhaps it’s fitting that one of Leakey’s greatest discoveries—rather, one of his wife’s greatest discoveries—is still contentious. In his day, some considered Leakey more of a showman than a scientist, but it’s hard to deny how his efforts furthered the study of human evolution.

The discoveries at Olduvai Gorge attracted other paleoanthropologists to East Africa, which is still the center of early-hominid research. Leakey’s son Richard was one of those researchers. In 1967, Leakey asked Richard to lead an archaeological expedition in Ethiopia. Richard eventually set out on his own and led the team that discovered the nearly complete Homo erectus skeleton called Turkana Boy. Richard’s wife, and Leakey’s daughter-in-law, Meave, was also a paleoanthropologist and helped discover Australopithecus anamensis (the earliest australopithcine species) and the engimatic Kenyanthropus platyops. Today, Louise Leakey, Leakey’s granddaughter, carries on the family’s homind-hunting tradition.

Leakey’s other great achievement was to help launch field studies of great apes. Leakey recognized the importance of studying ape behavior in the wild as a way to better understand the behavior of early hominids and other ancient apes. In 1960, he sent Jane Goodall to Gombe Stream National Park in Tanzania to study chimpanzees. In 1967, he helped Dian Fossey establish her fieldwork on the mountain gorillas living in the Virunga Volcanoes of Rwanda. And in 1971, he asked Biruté Galdikas to observe orangutans in Borneo. These three women were pioneers in living among primates as a way to study the animals’ natural behavior, and collectively were known as Leakey’s Ladies. (At least, that’s what I’ve always called them. According to Wikipedia, Leakey’s Angels is the preferred term.)

If I may be bold, I’ll call myself a second-generation Leakey Lady. When I was 12 years old, I watched the Dian Fossey biopic, Gorillas in the Mist, on TV. I decided at that moment that I wanted to study primates. Ten years later, I ended up in graduate school ready to do just that. That’s not what I ended up doing with my life. But here I am instead, writing a blog about human evolution. That never would have happened without Louis Leakey. And for that, I say, Happy Father’s Day, Dr. Leakey.

For a more in-depth look at Louis Leakey’s life, read Smithsonian’s “The Old Man of Olduvai Gorge” by Roger Lewin.

Sea otters have teeth that resemble those of Paranthropus. Image courtesy of Flickr user mikebaird

Fossils are the clues researchers study to better understand the history of life on earth. But to interpret those clues, scientists need to consider living animals. By looking at how the bones and physiology of modern creatures correlate with walking, eating, socializing and other habits, we can make inferences about what extinct animals with similar features might have been like.

In human evolution, hominids are most often compared to their living descendants—us. They are also compared to our closest living relative, the chimpanzee. This makes a lot of sense. We diverged from the chimpanzee lineage roughly seven million years ago; we share a lot of traits because we share a long evolutionary history.

But sometimes it’s more informative to compare hominids with more distantly related species that share traits due to convergent evolution—when two species evolve analogous characteristics, not because of common ancestry, but because of similar evolutionary pressures. The wings of bats and birds are one example; the fins of dolphins and sharks are another. Here are a few of my favorite examples of unexpected species that have played a role in the study of human evolution.

Sea Otters: These marine mammals don’t appear to have much in common with hominids, until they open their mouths. Sea otters have molars that resemble those of the genus Paranthropus, known for its giant jaw, massive chewing muscles and large molars with round cusps. Sea otters eat a lot of different foods, including critters with hard shells or outer skeletons; they can pop a clam, snail, sea urchin or crab into their mouths and crunch it whole. Researchers have long thought Paranthropus must have also eaten hard objects, perhaps nuts and seeds, in part because of its similarities with sea otters. Although recent research indicates these hominids may have spent much of their time grazing on tough plants such as grasses, rather than eating nuts, paleoanthropologists continue to study sea otters to see what they can learn about Paranthropus and other hominids.

Wolves: Wolves often come up in studies of human evolution, usually in discussions of dog domestication. But the social carnivore is useful in other ways. Adam Clark Arcadi, an anthropologist at Cornell University, used wolves to examine how many species of Homo might have had existed at one time. The question arises in relation to modern humans and Neanderthals: Were Neanderthals a separate species or just a subspecies of Homo sapiens? According to Arcadi, it’s likely there was only one human species. Even though regional populations might have developed different physical traits, a united species would have been maintained as long as there was some migration and mating between populations, what scientists call gene flow. Because humans are wide ranging and can live in a variety of habitats, he says, it’s likely gene flow was sustained.

As a way to think about the problem, Arcadi looked for another type of animal that is also wide-ranging and tolerant of numerous habitats—the wolf. Wolf packs can travel more than 100 miles per day; they can survive in deserts, forests, tundra and even urban areas; and they eat animals as big as caribou and as small as rodents, even munching on fruits or grass if they have to. The wolf analogy supports Arcadi’s case: The gray wolf, for example, traditionally lived throughout all of North America, Europe and Asia (before humans got in the way), yet it remained one species, Canis lupus. If the gray wolf can stay just one species, with about ten regional subspecies, Arcadi argues, then it’s also possible that there was just one species of Homo during the days of Neanderthals and modern humans.

Capuchin Monkeys: Unlike sea otters and wolves, capuchin monkeys may not seem like an unusual animal to compare hominids to. Yet in the primate world, more than 35 million years of evolution separate humans and capuchins. What they have in common are big brains and tool use. In Brazil, some populations of capuchins use sticks to probe holes and stones to hammer open palm nuts. Some researchers think we can learn more about how and why tool use evolved in hominids by exploring the differences between capuchin populations that use tools and those that don’t. One difference, noted by Eduardo Ottoni and Patricía Ozar of the University of São Paulo, Brazil (PDF), is the tool-using capuchins tend to be more terrestrial, living in savanna-like environments. Studying differences between tool-using and non-tool-using capuchins may also shed light on how tools affect social behavior.

Palm nuts must be a very nutritious and rewarding snack for the monkeys, because nutcracking appears to be very laborious. The cat-sized monkeys must lift what are to them boulder-size rocks up almost over their head and then pound them down on the nuts. The best way to appreciate a capuchin’s determination and skill is to watch one in action: