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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.

Small stone blades from South Africa dating to 71,000 years ago may be the earliest evidence of bow and arrows. Image: Simen Oestmo

The bow and arrow is an ancient weapon—going back at least 71,000 years, a study published in Nature suggests. Archaeologists working at South Africa’s Pinnacle Point cave site uncovered a collection of tiny blades, about an inch big, that resemble arrow points, likely belonging to prehistoric bow and arrows or spear-throwers. The researchers say the discovery is further evidence that humans (Homo sapiens) started to act and think like modern people early in their evolution.

The skeletons of H. sapiens appear in the fossil record by about 200,000 years ago in Africa. But when modern culture and cognition emerged is still an open question. Some anthropologists think the human brain evolved in tandem with the rest of the body, and culture built up slowly over time as technology advanced. Others have suggested there was a disconnect between physical and behavioral modernity, with some sort of genetic mutation roughly 40,000 years ago causing an abrupt change in how humans think. Still other researchers argue that incipient signs of advanced intellect appear early in the archaeological record but then disappear for thousands of years before reappearing. Needless to say, there’s a lot of debate on this subject. (For a detailed discussion on the topic, check out the story I wrote in June for Smithsonian.com).

Kyle Brown of the University of Cape Town and his colleagues say the tiny blades that they found are signs of complex tool making. The tiny tools were created from silcrete stone that people had heated over a fire to make the raw material easier to work with before chipping the rock into blades. This suggests people had to follow a lengthy multi-step process to make the blades, which included gathering the stones, gathering fuel for the fire, heating the rocks and carefully cutting the stone into delicate blades. The shape of the blades looks like the shape of arrow tips found in more recent arrows, which led Brown and colleagues to conclude the blades were used in bow-and-arrow projectile weapons. That implies there were even more steps in the tool-making process, such as hafting the stone tips to a wooden shaft.

The blades aren’t the only evidence that humans had advanced cognitive abilities as early as 71,000 years ago. Pigments, jewelry and other art found in South African cave sites dating to as many as 164,000 years ago suggest that early humans were capable of abstract or symbolic thinking. Some researchers view this ability as central to human intellect.

The new study, however, goes one step further. The researchers say the blades were found throughout a geological section of Pinnacle Point that spans roughly 11,000 years (71,000 to 60,000 years ago), indicating people could communicate complicated instructions to build intricate tools across hundreds of generations. This instance of long-term maintenance of a cultural tradition early in human history is evidence that the capacity for modern culture began early and slowly built up, Brown and colleagues say. Previous suggestions that complex culture came and went in the early days of humans is probably an artificial result, they say, because so few African sites have yet been excavated.

Humans share many traits with primates, such as these Barbary macaques, including excellent vision and great dexterity. Image: markhsal/Flickr

I’m a primate. You’re a primate. Everyone reading this blog is a primate. That’s not news. We hear it all he time: Humans are primates. But what does that really mean? What do we have in common with a baboon? Or a creepy aye-aye? Or even our closest living relative, the chimpanzee?

These are simple questions to answer from a genetic perspective—humans share more DNA with lemurs, monkeys and apes than they do with other mammals. Genetic research of the last few decades suggests that humans and all living primates evolved from a common ancestor that split from the rest of the mammals at least 65 million years ago. But even before DNA analyses, scientists knew humans belong in the primate order. Carl Linnaeus classified humans with monkeys, apes and other primates in his 18th-century taxonomic system. Even the ancient Greeks recognized similarities between people and primates. Today, anthropologists recognize several physical and behavioral traits that tie humans to primates.

Primates have nimble hands and forward-facing eyes, as this capuchin monkey demonstrates. Image: Tambako the Jaguar/Flickr

First, primates have excellent vision. They have forward-facing eyes that sit close together, which allows the eyes’ fields of view to overlap and create stereoscopic, or 3-D, vision. (In contrast, for example, a cow or giraffe has widely spaced eyes and therefore poor depth perception.) Related to this great eyesight is the presence of a post-orbital bar, a ring of bone that surrounds the eyeball. Many primates also have a completely bony socket that encloses the eye. This bone probably protects the eye from contractions of chewing muscles that run down the side of the face, from the jaw to the top of the head. Many mammals that rely less on vision don’t have a post-orbital bar. If you poked a dog in the side of its head near the temple, you would feel muscle and the eye but no bone (and you would probably be bitten, so please don’t do that). Because primates depend on their vision so much, they generally have a reduced sense of smell relative to other mammals.

Primates are also very dexterous. They can manipulate objects with great skill because they have opposable thumbs and/or big toes, tactile finger pads and nails instead of claws (although some primates have evolved so-called grooming claws on some of their toes). Primates also generally have five fingers/toes on each hand/foot. This is actually a very ancient trait. The earliest mammals had five digits, and over time, many mammalian lineages lost a few fingers and toes while primates kept all of them. Primates also retain collar bones, which allow for greater mobility in the shoulder; mammals that strictly walk on all fours, such as horses, lack collar bones so their limbs are more stable and don’t slip to the side while running.

And in general, primates tend to have larger brains than other mammals of a similar size. They also have smaller litters—often just one baby at a time—and longer periods of gestation and childhood.

Scientists are still trying to understand why primates’ unique set of features evolved. Some researchers think the earliest primates lived in trees, so good vision and dexterity would have been helpful in judging distances between branches or for climbing around. Others, such as Boston University’s Matt Cartmill, have suggested that these traits emerged because early primates might have been insect predators and needed clear eyesight and quick hands to grab prey. Both factors, as well as many others, could have played a role.

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.