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A partial Homo antecessor skull that was unearthed at the Gran Dolina cave site in the Atapuerca Mountains of  Spain. Image: José-Manuel Benito/Wikicommons

Humans and Neanderthals split from a common ancestor roughly half a million years ago. While many anthropologists will tell you we don’t really know who that common ancestor was, others will say we do: the species Homo heidelbergensis, or something very much like it. An even smaller portion will point to another possibility: a controversial species called Homo antecessor.

H. antecessor, which first came to light in the 1990s, is known almost entirely from one cave in northern Spain’s Atapuerca Mountains. While working at the Gran Dolina site from 1994 to 1996, a team of Spanish researchers found 80 fossils belonging to six hominid individuals that lived roughly 800,000 years ago. The hominids’ teeth were primitive like those of Homo erectus, but aspects of the hominid’s face—particularly the shape of the nasal region and the presence of a facial depression above the canine tooth called the canine fossa—were modern, resembling features of modern people. The unique mix of modern and primitive traits led the researchers to deem the fossils a new species, H. antecessor, in 1997.

In 2008, the researchers expanded the timeline of the species . At another cave site in Atapuerca, Sima del Elefante, scientists unearthed a partial lower jaw, as well as a few dozen stone tools, dating to about 1.2 million years ago. Outside of Spain, the only other potential evidence of H. antessor fossils are stone tools found at a nearly 800,000-year-old English archaeological site named Happisburgh that might have been made by the species.

H. antessor‘s discoverers—including José Bermúdez de Castro of Spain’s National Museum of Natural Sciences, Juan Luis Arsuaga of the Universidad Complutense in Madrid and Eudald Carbonell of the University of Tarragona—say the species’ similarities with modern people, and its age, make it the best known candidate for the common ancestor of Neanderthals and Homo sapiens. They suggest H. antecessor may have evolved from a population of H. erectus living in Africa more than 1.5 million years ago and then migrated to Europe, journalist Ann Gibbons reported in Science when H. antecessor was first announced. Although the species has yet to be discovered in Africa, an African origin for H. antecessor may be necessary if it was indeed the direct ancestor of modern humans, which all fossil evidence suggests originated in Africa. Furthermore, the researchers say H. heidelbergensis is too similar to Neanderthals to be a direct ancestor of modern humans. Instead, H. antecessor gave rise to H. heidelbergensis, which then gave rise to Neanderthals.

But many anthropologists are not on board with this scenario. One problem is that most of the known H. antecessor specimens represent children, Gibbons reported. Only two of the six individuals found at Gran Dolina are thought to be adults, about 20 years old. Since most of the features tying H. antecessor to modern people were found in juveniles—whose bodies and physical features change as they grow up and go through puberty—it’s possible that H. antecessor adults didn’t really look much like H. sapiens at all. And if that’s the case, then it’s hard to argue the species had an ancestor-descendent relationship with us. The issue won’t be settled until researchers find good examples of complete adult H. antecessor fossils.

An artist’s reconstruction of Paranthropus boisei, a hominid species that was first discovered in Tanzania. Image: dctim1/Flickr

Lucy and Ardi are the poster children of human evolution. But these famous fossil skeletons may never have been found if it weren’t for Louis and Mary Leakey’s pioneering efforts. The pair made several discoveries at Tanzania’s Olduvai Gorge in the 1950s and 1960s that inspired other anthropologists to come to East Africa in search of human ancestors. Here’s a look at some of the most important hominid fossil finds from Tanzania.

The Nutcracker Man (OH 5): The Leakeys’ first major discovery at Olduvai Gorge occurred in 1959. Mary found the roughly 1.8-million-year-old skull of a hominid with a flat face, gigantic teeth, a large crest on the top of its head (where chewing muscles attached) and a relatively small brain. They named the species Zinjanthropus boisei (now known as Paranthropus boisei). Nicknamed the Nutcracker Man, the species was too different from modern people to be the direct human ancestor that Louis had been hoping to find. But the discovery captured public interest in human evolution, and the Leakeys went on to unearth many more hominid fossils at Olduvai. OH 5 is the fossil’s official catalog name, meaning Olduvai Hominid Number 5.

Johnny’s Child (OH 7): The next big Leaky discovery came in 1960. Mary and Louis’ son, Johnny, found a lower jaw about 300 yards away from where the Nutcracker Man was discovered. The bone came from a young hominid; thus, the fossil was nicknamed Johnny’s Child. At the same spot, the Leakeys also dug up some hand bones and skull fragments. Using these skull fragments, the Leakeys and their colleagues estimated the roughly 1.8-million-year-old hominid’s brain size: 680 cubic centimeters. That was significantly bigger than the size of the average australopithecine brain, about 500 cubic centimeters. The hand bones revealed that the hominid had a “precision grip,” when a fingertip presses against the tip of the thumb. This movement allows for fine manipulation of objects, such as turning a key in a door or threading a needle. The precision grip led the Leakeys to conclude that this hominid was the one who made the stone tools found at Olduvai. Because of the tool-making and the big brain, the Leakeys decided OH 7 represented the earliest member of the genus Homo: Homo habilis (meaning Handy Man).

OH 8: Also in 1960, the Leakeys’ team discovered a well-preserved fossil foot belonging to H. habilis. The bones indicate the hominid had modern-looking foot arches, suggesting the species walked like modern people do. Tooth marks on the specimen’s ankle reveal the hominid had been a crocodile’s lunch.

OH 9: At the same time the Leakeys unearthed the first examples of H. habilis, they also recovered the skull cap of a more recent hominid dating to about 1.4 million years ago. At 1,000 cubic centimeters, the specimen’s brain was much bigger than that of H. habilis. The skull had thick brow ridges and a low, sloped forehead—key features linking the fossil to the species Homo erectus.

Twiggy (OH 24): Discovered in 1968 by Peter Nzube, Twiggy is a skull belonging to an adult H. habilis dating to roughly 1.8 million years ago. Although OH 24 is the most complete H. habilis skull from Olduvai Gorge, it was found crushed completely flat (and therefore named after the slender British model of the same name). Paleoanthropologist Ron Clarke reconstructed what the skull would have looked like, but it’s still fairly distorted.

LH 4: In the 1970s, after Louis died, Mary began excavations at Laetoli, about 30 miles from Olduvai Gorge. The fossils she was finding there were much older than the bones she and Louis had discovered at Olduvai. In 1974, for example, her team unearthed a lower jaw with teeth dating to 3.6 million years ago. It was cataloged as Laetoli Homind 4, or LH 4. Around the same time, anthropologists at the site of Hadar in Ethiopia were also finding hominid fossils dating to more than 3 million years ago, including the famous Lucy skeleton. At first, no one was sure what to call these older fossils. After analyzing both the Hadar and Laetoli specimens, anthropologists Tim White and Donald Johanson (Lucy’s discoverer) concluded that all of the fossils represented one species that they called Australopithecus afarensis. They chose LH 4 as the species’ type specimen, or the standard representative of the species. Mary did not approve. She didn’t believe the fossils from Laetoli were australopithecines. But under the rules of taxonomy, once a type specimen is designated, it’s forever associated with its species name. (For more on the controversy, see Johanson’s book Lucy.)

Laetoli Footprints: In 1978, one of Mary’s team members, Paul Abell, made the most famous discovery at Laetoli: He found the trail of about 70 fossilized hominid footprints. Based on the footprints’ age, 3.6 million years, anthropologists think they were made by an A. afarensis group. The footprints reveal this early hominid had a very modern way of walking. The big toe was in line with the other toes, not off to the side like an ape’s big toe. And the prints reveal the walkers had arches, unlike the flat feet of an ape. The footprints also suggest A. afarensis had a modern gait.

A reconstruction of Lucy, an Australopithecus afarensis. Lucy probably walked much slower than taller members of her species. Image: Travis S./Flickr

If you’re on the shorter end of the height spectrum, you know how frustrating it can be to take a stroll with someone who’s tall. At times, you might have to remind your companion to slow down, that your shorter legs can’t keep up. This might have been an even bigger problem for our famous ancestor, Lucy. Within the species Australopithecus afarensis, there was considerable variability in height and limb length, and different members of the species may have had vastly different preferences for walking speeds, new research suggests. How did our ancestors cope with such a dilemma?

The problem really became apparent in 2010 with the discovery of a partial A. afarensis skeleton, nicknamed “Big Man,” in Ethiopia. As his name suggests, the five-foot-tall Big Man was big, at least for an early hominid, and compared to the three-and-a-half-foot-tall Lucy. Big Man’s shin, for instance, was about 50 percent longer than that of Lucy’s—the sort of length difference you see today between a six-year-old child and a six-foot-tall man. But in Lucy and Big Man’s case, both individuals were adults, suggesting there was a large range of heights for A. afarensis. The variation might have been related to sex, with males being significantly taller than females. Or there might have been regional differences in A. afarensis size. Lucy and Big Man were both found in Ethiopia but at different sites.

To understand the walking behavior of Lucy, Big Man and their kind, Patricia Ann Kramer of the University of Washington in Seattle did some experiments with people. In modern humans, the length of the lower leg (or tibia) plays a big role in how much energy a person expends while walking and what his/her preferred speed is. Kramer examined this relationship by measuring the tibia length of 36 children and 16 adults and then placing the volunteers on treadmills to record how much energy they used (measured in terms of oxygen consumption) while walking at different speeds. She discovered that, in general, individuals with longer lower legs have higher “optimal velocities.” That means the speed at which longer-legged people consume the least amount of energy is faster than that of shorter-legged people.

Kramer used the data to create a mathematical equation that related leg length to speed to estimate Lucy’s and Big Man’s optimal velocities based on their tibia lengths. Lucy’s would have been 1.04 meters per second (about 3.4 feet per second) while Big Man’s would have been as much as 1.33 meters per second (about 4.4 feet per second). To put this in perspective, if both individuals walked for an hour at their optimal speeds, Lucy would have covered 3.74 kilometers (2.3 miles) while Big Man would have traversed 4.68 kilometers (2.9 miles), Kramer reports in the American Journal of Physical Anthropology.

Based on two individuals, it’s hard to say how representative these results are for A. afarensis. And even assuming there were big differences in walking speeds, it’s hard to say how it would have affected the behavior of these early hominids. If size differences were sex based, then some members of a group might have had to compromise their preferred walking speed—perhaps females had to walk faster (and thus expend more energy) to keep up with males or maybe males slowed down (also expending more energy) to appease females or maybe both sexes had to adjust their velocities. Another possibility is that males and females spent time away from each other during the day, Kramer says. Among wild chimpanzees, males and females often range separately while searching for food, which might be a consequence of different walking speeds. More studies that examine sex-based ranging patterns in primates might offer more clues to how A. afarensis could have coped. Of course, this variation in  height might not have been a problem at all if differences were largely regional.

Although Kramer’s work doesn’t provide any definite answers, it highlights how difficult it is to reconstruct the biology and behavior or our ancestors. It’s clear that A. afarensis walked upright, but we still have a lot to learn about how the early hominid traveled across the East African landscape.

An artist’s reconstruction of Purgatorius, a probable primate ancestor. Image: Nobu Tamura/Wikicommons

Finding the earliest primates isn’t easy. The first members or our order probably lived about 65 million years ago and were rat-sized critters known mainly from teeth. With such scant evidence, researchers have had a hard time classifying these creatures and making connections to modern primates. Still, scientists have identified dozens of early primate, or probable primate, species. If you’re unfamiliar with our earliest origins, here are five primates to know.

Purgatorius: Discovered at Montana’s Hell Creek Formation, this shrew-sized mammal lived roughly 65 million years ago at the end of the Cretaceous period. Purgatorius‘ place in the primate family tree is debated. Aspects of the genus’ teeth align it with a group of extinct, primate-like mammals called plesiadapiforms. Some scientists say that the number and variety of teeth Purgatorius had makes it a possible common ancestor to primates and plesiadapiforms. Last week, paleontologists from Yale University announced they found  the first known Purgatorius ankle bones. The researchers say the fossils reveal the animal had flexible feet like modern tree-living mammals do, implying the earliest primates were indeed arboreal animals as scientists suspected.

Altiatlasius: A few molars and a jaw fragment are all that’s known of this small mammal discovered in Morocco. Many paleontologists consider Altiatlasius, which lived some 57 or 56 million years ago, to be the first true primate. How the ancient primate relates to modern primate lineages is unclear. While some researchers believe it’s similar to a group of primitive tarsier-like primates, others think it might be an ancient forefather of monkeys and apes.

Teilhardina: Named for the French paleontologist Pierre Teilhard de Chardin, Teilhardina has been found at North American and Asian sites dating to almost 56 million years ago. Scientists group the genus with the omomyids, a family of tarsier-like primates that emerged during the Eocene epoch some 56 million to 34 million years ago. Last year, scientists reported they had unearthed a cache of Teilhardina fossils in Wyoming’s Big Horn Basin that included the first evidence that early primates had nails instead of claws. The tips of the animal’s finger and toe bones were flattened, indicating the presence of fingernails, the researchers reported in the American Journal of Physical Anthropology.

Notharctus: This North American genus lived about 50 million years ago and belonged to a family of lemur-like primates called adapiforms. Notharctus had a long tail, leaped from tree to tree and snacked on leaves. A report published in PLOS ONE in January described fossils from this primate that indicate it would have had something like a cross between a fingernail and a claw on its second toe—kind of like modern lemurs, lorises and bush babies (or galagos) that all have a “grooming” claw on their second toe. But it’s not yet clear whether Notharctus was on its way towards evolving a true grooming claw, or on its way towards evolving a true nail.

Eosimias: Discovered in China, Eosimias lived about 45 million years ago. The size and shape of its teeth suggest it was the earliest ancestor of the lineage leading to monkeys and apes (and us!). Fossils of its feet suggest Eosimias walked on all fours like a modern monkey.

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.