Fossils discovered in Kenya indicate multiple species of Homo lived roughly two million years ago. One of the new jaws is pictured here with a previously found Homo rudolfensis skull. Image: © Photo by Fred Spoor

As 2012 nears its end, one thing stands out as the major theme in human evolution research this year: Our hominid ancestors were more diverse than scientists had ever imagined. Over the past 12 months, researchers have found clues indicating that throughout most of hominids’ seven-million-year history, numerous species with a range of adaptations lived at any given time. Here are my top picks for the most important discoveries this year.

1. Fossil foot reveals Lucy wasn’t alone: Lucy’s species, Australopithecus afarensis, lived roughly 3.0 million to 3.9 million years ago. So when researchers unearthed eight 3.4-million-year-old hominid foot bones in Ethiopia, they expected the fossils to belong to Lucy’s kind. The bones do indicate the creature walked upright on two legs, but the foot had an opposable big toe useful for grasping and climbing. That’s not something you see in A. afarensis feet. The researchers who analyzed the foot say it does resemble that of the 4.4-million-year-old Ardipithecus ramidus, suggesting that some type of Ardipithecus species may have been Lucy’s neighbor. But based on such few bones, it’s too soon to know what to call this species.

2. Multiple species of early Homo lived in Africa: Since the 1970s, anthropologists have debated how many species of Homo lived about two million years ago after the genus appeared in Africa. Some researchers think there were two species: Homo habilis and  Homo rudolfensis; others say there was just H. habilis, a species with a lot of physical variation. It’s been a hard question to address because there’s only one well-preserved fossil, a partial skull, of the proposed species H. rudolfensis. In August, researchers working in Kenya announced they had found a lower jaw that fits with the previously found partial skull of H. rudolfensis. The new jaw doesn’t match the jaws of H. habilis, so the team concluded there must have been at least two species of Homo present.

3. New 11,500-year-old species of Homo from China: In March, researchers reported they had found a collection of hominid bones, dating to 11,500 to 14,300 years ago, in a cave in southern China. Based on the age, you’d expect the fossils to belong to Homo sapiens, but the bones have a mix of traits not seen in modern humans or populations of H. sapiens living at that time, such as a broad face and protruding jaw. That means the fossils may represent a newly discovered species of Homo that lived side by side with humans. Another possibility is that the remains came from Denisovans, a mysterious species known only from DNA extracted from the tip of a finger and a tooth. Alternatively, the collection may just reveal that H. sapiens in Asia near the end of the Pleistocene were more varied than scientists had realized.

4. Shoulder indicates A. afarensis climbed trees: Another heavily debated question in human evolution is whether early hominids still climbed trees even though they were built for upright walking on the ground. Fossilized shoulder blades of a 3.3-million-year-old A. afarensis child suggest the answer is yes. Scientists compared the shoulders to those of adult A. afarensis specimens, as well as those of modern humans and apes. The team determined that the A. afarensis shoulder underwent developmental changes during childhood that resemble those of chimps, whose shoulder growth is affected by the act of climbing. The similar growth patterns hint that A. afarensis, at least the youngsters, spent part of their time in trees.

5. Earliest projectile weapons unearthed: Archaeologists made two big discoveries this year related to projectile technology. At the Kathu Pan 1 site in South Africa, archaeologists recovered 500,000-year-old stone points that hominids used to make the earliest known spears. Some 300,000 years later, humans had started making spear-throwers and maybe even bow and arrows. At the South African site called Pinnacle Point,  another group of researchers uncovered tiny stone tips dated to 71,000 years ago that were likely used to make such projectile weapons. The geological record indicates early humans made these small tips over thousands of years, suggesting people at this point had the cognitive and linguistic abilities to pass on instructions to make complex tools over hundreds of generations.

6. Oldest evidence of modern culture: The timing and pattern of the emergence of modern human culture is yet another hotly contested area of paleoanthropology. Some researchers think the development of modern behavior was a long, gradual buildup while others see it as progressing in fits and starts. In August, archaeologists contributed new evidence to the debate. At South Africa’s Border Cave, a team unearthed a collection of 44,000-year-old artifacts, including bone awls, beads, digging sticks and hafting resin, that resemble tools used by modern San culture today. The archaeologists say this is the oldest instance of modern culture, that is, the oldest set of tools that match those used by living people.

7. Earliest example of hominid fire: Studying the origins of fire is difficult because it’s often hard to differentiate a natural fire that hominids might have taken advantage of versus a fire that our ancestors actually ignited. Claims for early controlled fires go back almost two million years. In April, researchers announced they had established the most “secure” evidence of hominids starting blazes: one-million-year-old charred bones and plant remains from a cave in South Africa. Because the fire occurred in a cave, hominids are the most likely cause of the inferno, the researchers say.

8. Human-Neanderthal matings dated: It’s not news that Neanderthals and H. sapiens mated with each other, as Neanderthal DNA makes up a small portion of the human genome. But this year scientists estimated when these trysts took place: 47,000 to 65,000 years ago. The timing makes sense; it coincides with the period when humans were thought to have left Africa and spread into Asia and Europe.

9. Australopithecus sediba dined on wood: Food particles stuck on the teeth of a fossil of A. sediba revealed the nearly two-million-year-old hominid ate wood—something not yet found in any other hominid species. A. sediba was found in South Africa in 2010 and is a candidate for ancestor of the genus Homo.

10. Earliest H. sapiens fossils from Southeast Asia: Scientists working in a cave in Laos dug up fossils dating to between 46,000 and 63,000 years ago. Several aspects of the bones, including a widening of the skull behind the eyes, indicate the bones were of H. sapiens. Although other potential modern human fossils in Southeast Asia are older than this find, the researchers claim the remains from Laos are the most conclusive evidence of early humans in the region.

The Little Foot skeleton embedded in a cave at Sterkfontein. Image © Maropeng

South Africa plays a central role in the history of paleoanthropology. Anthropologists and other scientists of the 19th and early 20th century balked at the possibility that Africa was humankind’s homeland—until an ancient hominid was unearthed in South Africa in 1924. Since then, Africa has become the center of human evolution fieldwork, and South Africa has produced a number of iconic hominid fossils and artifacts. Here is a totally subjective list of the country’s most important hominid discoveries.

Taung Child: In 1924, anatomist Raymond Dart pried a tiny fossilized partial skull and brain from a lump of rock. The bones were the remains of a child. The youngster looked like an ape, but Dart also recognized some human qualities. He decided he had found a human ancestor that was so ancient it was still ape-like in many ways. (Later, scientists would determine the bones were nearly three million years old). Dart named the hominid Australopithecus africanus. The Taung Child, known by the name of the place where the fossils came from, was the first australopithecine ever discovered—and the first early hominid found in Africa. After the discovery, anthropologists who were searching for humanity’s origins in Europe and Asia switched their attention to Africa.

Mrs. Ples: Throughout the 1930s and 1940s, paleontologist Robert Broom led the efforts to find hominids in South Africa. He scoured the region’s limestone caves and quarries—the Taung Child came from a quarry—and was well rewarded for his efforts. Of the numerous fossils he uncovered (sometimes with the help of dynamite), his most influential find was a roughly 2.5-million-year-old skull of an adult female hominid now known as Mrs. Ples. Unearthed in 1947 at a site called Sterkfontein, the skull was well preserved and displayed the same mix of ape and human features seen in the Taung Child. Finding an adult version of A. africanus helped convince skeptics that the species was an ancient human ancestor. Some anatomists had thought Taung was just an ape and would have developed more pronounced ape-like features, and lost its human-like traits, as it grew up. Instead, Mrs. Ples showed that the species retained its mix of human and ape traits throughout life.

STS 14: Another one of Broom’s key finds is a set of well-preserved post-cranial bones that includes a pelvis, partial spine, ribs and upper thigh. Like Mrs. Ples, these fossils were found in 1947 at Sterkfontein and date to about 2.5 million years ago. The bones are officially known as STS 14 (STS refers to Sterkfontein) and presumably belonged to an A. africanus individual. The shape of the pelvis and spine are remarkably modern, and the find was some of the first evidence that early human ancestors walked upright on two legs.

SK 48: In addition to finding a trove of A. africanus specimens, Broom, along with his many assistants, discovered a new hominid species: Paranthropus robustus. The first hints of the species came in 1938 when Broom acquired a jaw fragment and molar that were much larger and thicker than any fossils belonging to A. africanus. Broom collected more of the unusual fossils and then hit the jackpot in 1950. A quarry worker found a nearly complete skull of an adult hominid that had giant teeth and a flat face. The fossil is officially called SK 48 (SK refers to the cave of Swartkrans where the skull was found). The collection of fossils with big chompers, which the hominids used to chew tough foods, was given the name P. robustus, which lived in South Africa about 1.8 million to 1.2 million years ago.

Little Foot: In the early 1990s, anthropologist Ron Clarke of South Africa’s University of the Witwatersrand found four small australopithecine foot bones at Sterkfontein. Later, Clarke and his colleagues discovered a nearly complete skeleton embedded in limestone that belonged to the foot. The researchers are still carefully chipping away at the rock to release the skeleton, dubbed Little Foot, but they have already noted that the individual has some characteristics not seen in any other known species of Australopithecus. But since the bones haven’t been fully studied and shared with other scientists, it’s hard to know where the hominid sits in the family tree, Science reported last year. It’s also hard to know exactly how old it is. Clarke’s team places the fossils at 3.3 million years old while other groups using different dating methods say Little Foot is more like 2.2 million years old. Science reported that Little Foot was expected to be fully liberated from its rocky enclosure sometime this year. As far as I know, that hasn’t happened yet.

Australopithecus sediba: The most recent major hominid fossil discovery in South Africa occurred in 2010. Lee Berger of the University of the Witwatersrand led a team that found two partial hominid skeletons at Malapa Cave. Dating to nearly two million years ago, the skeletons indicate that these hominids had their own unique style of walking and spent time both on the ground and in trees. X-ray scans of one of the skulls reveals that some aspects of the brain were more modern than in previous species. Berger and his colleagues therefore think the species, which they named A. sediba, could have given rise to the genus Homo.

Origins of Modern Behavior: Fossils aren’t the only major human evolution discoveries from South Africa. Several coastal cave sites have been treasure troves of artifacts that reveal when and how sophisticated behavior and culture emerged in early populations of Homo sapiens. There have been too many of these discoveries to single any one out. Some of these finds—such as red pigments used 164,000 years ago and shell beads dating to 77,000 years ago—are among the earliest evidence for symbolic thinking in our ancestors. Other artifacts, like 71,000-year-old projectile weapons, indicate early humans could construct complicated, multipart tools that require a lot of planning and foresight to make.

The 3.5-million-year-old Australopithecus bahrelghazali from Chad probably ate grass, just like the modern baboons seen here do. Image: GregRob/Flickr

The nearly 2-million-year-old Paranthropus boisei was the cow of the hominid family. Unlike other human cousins, the species was a fan of dining on grasses. But it turns out it wasn’t the only, or even the first, hominid grazer. Australopithecus bahrelghazali was munching on grasses and sedges at least 1.5 million years before the origin of P. boisei, a new study in the Proceedings of the National Academy of Sciences suggests. The findings may mean early hominids were capable of consuming a wide variety of foods and colonizing new environments.

But before we discuss how scientists figured out A. bahrelghazali‘s diet, and why that matters, we need to address a far more pressing question: Who the heck was A. bahrelghazali?

In 1993, researchers in Chad unearthed a 3.5-million-year-old hominid lower jaw fragment and a few attached teeth. Based on the fossils’ age, many paleoanthropologists think the bones belonged to Australopithecus afarensis. But the specimen was found more than 1,500 miles farther west than any other A. afarensis bones, and subtle differences in the size and shape of the fossils led the discoverers to conclude they had found a new species. They named it A. bahrelghazali after the Bahr el Ghazal valley in Chad where the bones were recovered. Since then, researchers haven’t found any other  A. bahrelghazali fossils and its species’ status remains controversial.

With just a jaw and teeth, there’s not too much scientists can say about what A. bahrelghazali looked like or how it lived its life. But, fortunately, diet is something that can be gleamed from these fossils. Analyzing the teeth’s chemistry is one way to assess what the species ate. This is possible because the carbon found in plants comes in two versions, or isotopes, called C3 and C4. Trees and other forest plants are rich in C3; grasses, sedges and other grassland plants have an abundance of C4. When an animal eats these plants—or eats other animals that eat these plants—the different carbon isotopes get incorporated into the individual’s teeth, serving as a record of what it once ate. Previous work on P. boisei has shown that C4 plants made up as much as 77 percent of that hominid’s diet.

In the new study, Julia Lee-Thorp of Oxford University and colleagues come to a similar conclusion for A. bahrelghazali, that the species mainly ate C4 plants, probably grasses and sedges. And like modern baboons that live on savannas, the hominid probably ate different parts of these plants, including underground tubers and bulbs. This diet is not surprising given the type of habitat A. bahrelghazali lived in. Based on the other types of animals found near the hominid, the researchers say A. bahrelghazali made its home in an open grassland, with few trees, near a lake. So forest foods weren’t really a dining option.

The results mean that by 3.5 million years ago hominids were probably already “broad generalists” capable of eating a variety of foods depending on what was locally available, the researchers say. (The younger Australopithecus sediba,which lived roughly 2 million years ago, demonstrates some of the stranger foods that hominids could eat: The South African species liked to eat wood—a dietary preference not seen in any other hominid.) Being a food generalist may have allowed A. bahrelghazali to explore new environments and leave behind the forests that earlier hominids, such as Ardipithecus ramidus, and their ancestors resided in.

The 3.3-million-year-old fossils of an Australopithecus afarensis child from Dikika, Ethiopia, suggest the hominid climbed trees. The individual’s right shoulder blade (side view) is visible beneath the skull. Image: Courtesy of Zeresenay Alemseged/Dikika Research Project

The most famous Australopithecus afarensis skeleton is named for the Beatles’ “Lucy in the Sky with Diamonds.” But a better anthem for the species might be “Lucy in the Trees with Chimpanzees.” A new study investigating how A. afarensis‘ shoulders grew during childhood indicate the early hominid spent at least some of its time climbing in trees. The work, published online today in Science, adds another bit of evidence to a decades-long debate about how Lucy and her kind traveled through their environment.

There’s no question that A. afarensis, which lived about 3.85 million to 2.95 million years ago, walked upright on two legs. The species possessed numerous physical features associated with bipedalism, such as thighs that angled in toward the knees and arched feet that lacked the grasping big toes seen in tree-climbing apes. But the hominid also had characteristics that are normally found in arboreal apes, such as curved fingers and toes, which are useful for gripping tree limbs. So the controversial question has been: Did A. afarensis actually climb trees? Or were the so-called climbing traits just evolutionary holdovers that the species didn’t use but hadn’t lost yet?

The new study takes a novel route in addressing these questions, looking at the development of the shoulder blades in A. afarensis. David Green of Midwestern University in Downers Grove, Illinois, and Zeresenay Alemseged of the California Academy of Sciences began by carefully liberating the left and right shoulder blades from the block of rock holding together the Dikika Child, a 3-year-old A. afarensis that lived about 3.3 million years ago. The fossil was unearthed in Ethiopia between 2000 and 2003, and it’s taken this long to remove the delicate shoulder blades, which are a rare find in the hominid fossil record.

The tiny right shoulder blade after it was removed from the rest of the Dikika Child’s fossils and rock encasement. Image: Courtesy of David J. Green

The pair compared the Dikika Child’s shoulder bones with those of a few adult A. afarensis specimens, as well as those of juvenile and adult shoulders from other Australopithecus species, Homo erectus, modern humans and modern apes. By comparing children to adults, the researchers could assess how the size and shape of the shoulder blade changed as a young A. afarensis grew up. In chimpanzees and gorillas, the shoulder blade develops in a characteristic way because frequent climbing during childhood affects how the shoulder grows—in other words, the apes’ shoulders change as a result of climbing. The shoulders of modern humans and H. erectus look very different and have their own growth trajectory because neither species spends any significant time climbing during childhood and adolescence (playing on “monkey” bars doesn’t count). In the new research, Green and Alemseged conclude the shoulder of A. afarensis developed in the same manner as an African ape’s, indicating the early hominid must have spent at least some time climbing in trees.

That doesn’t mean swinging through the treetops was A. afarensis‘ preferred mode of locomotion. In the past, paleoanthropologists have suggested that Lucy’s small size (she was no bigger than a chimp) made her vulnerable to leopards and other hungry predators. So while the hominid might have spent most of its time walking upright on the ground, at night it might have taken shelter in trees—perhaps making a nest as many chimpanzees do.

The 3.5-million-year-old skull of Kenyanthropus platyops. Image: Pavel Švejnar/Wikicommons

Three and a half million years ago was the heyday of Australopithecus afarensis. But Lucy’s species may not have been alone. In 1999, researchers working in West Turkana, Kenya, uncovered a roughly 3.5-million-year-old hominid skull with a face too flat to belong to A. afarensis. The skull’s finders decided it must be a new species, Kenyanthropus platyops.

At the time, anthropologists disagreed over K. platyops‘ identity and place in the human family tree. While some researchers take the species as a sign that there was a diversity of hominid types around during the middle Pliocene epoch, others say the K. platyops skull is not actually a distinct species at all—it’s simply a distorted skull of an A. afarensis.

Where does the debate stand today?

In 2010, some of the original describers of K. platyops—Fred Spoor of the Max Planck Institute for Evolutionary Anthropology in Germany and Meave Leakey of the Koobi Fora Research Project—worked with Koobi Fora’s Louise Leakey to reanalyze the skull. In their new analysis, the team used CT scans of the skull to assess how distorted the fossil really is. Although the skull contains numerous cracks, the shape of the skull and teeth remain largely unaffected by the damage, the researchers reported in Proceedings of the Royal Society B.

With that in mind, Spoor and the two Leakeys compared the physical features of K. platyops to those of A. afarensis and six other extinct hominid species, in addition to modern humans, chimpanzees and gorillas. According to the researchers, the results confirmed that K. platyops was significantly different from other hominid species, mainly in its flat face, forward-facing cheekbones and small molars (over time, the molars got bigger and bigger in the many species of Australopithecus before getting small again in the genus Homo). Thus, the fossil deserves to be in its own species, they concluded.

Of course, one paper never really settles a debate in human evolution. The Smithsonian Human Origins Program, for example, doesn’t include K. platyops in its list of hominid species and categorizes the K. platyops skull as A. afarensis. Many scientists probably won’t be convinced unless more fossils matching K. platyops are found. There are some other fragments of teeth and skull from West Turkana that may belong to the species, but those fossils don’t really shed any further light on the issue.

For those who do accept K. platyops as a distinct hominid, there’s not much to say about the species. Given its age, it’s possible that K. platyops made the famous footprints preserved at Laetoli in Tanzania. The footprints reveal that some sort of hominid with a modern gait was alive at the time. The more popular theory, however, is that A. afarensis was the upright walker at Laetoli.

Regardless of K. platyops‘ status as a species, it’s not the only evidence that more than one type of hominid lived in East Africa alongside Lucy. In April, researchers unearthed a 3.4-million-year-old hominid foot in Ethiopia that retained too many features related to climbing to be the foot of  A. afarensis. Who knows—maybe it belonged to K. platyops.