Smithsonian.com

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.

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.

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.

Dating and mapping fossil finds is one way anthropologists track early human migrations. The bones from Qafzeh, Israel, (a drawing of one of the skulls, above) indicate Homo sapiens first left Africa more than 100,000 years ago. Image: José-Manuel Benito/Wikicommons

By 200,000 years ago, Homo sapiens had emerged somewhere in Africa. By 14,000 years ago, our species had spread to every continent except Antarctica. What happened in between—the pattern of where humans went and when—is still being worked out. To reconstruct the peopling of the world, anthropologists rely on several types of clues.

Fossils: The most obvious way to track our ancestors’ movements is to look for their physical remains. Researchers sketch out travel routes by mapping where the oldest human fossils are found. The earliest Homo sapiens bones outside of Africa come from a cave site in Israel called Qafzeh. Here the skeletons of both adults and children date to as far as 125,000 years ago. This first foray out of Africa didn’t last long. Humans disappeared from the fossil record outside of Africa for many tens of thousands of years, perhaps because the climate became too harsh. Fossils tell us humans made a successful, sustained exodus by at least 50,000 years ago. Human fossils found at Australia’s Lake Mungo site, for example, have been dated to between 46,000 and 50,000 years ago (PDF).

The problem with relying on skeletal remains to map early migrations is that the timing of our ancestors’ travels is only as good as the methods used to date the fossils. Sometimes scientists find bones in places that are not easily dated by geological techniques. And in some areas, fossils aren’t prone to preservation, so there are probably huge gaps in our knowledge of the paths early humans took as they spread around the world.

Artifacts: Archaeologists also look for the items people made and left behind. For example, stone tool discoveries suggest an alternative route out of Africa. For decades, scientists assumed humans left Africa via the Sinai Peninsula, but in the last several years some researchers have favored a “southern” route: leaving from the Horn of Africa, crossing the narrowest part of the Red Sea and entering into southern Arabia. Last year, archaeologists reported finding stone tools in Oman dating to roughly 106,000 years ago. At that time, the Arabian Peninsula was a much more hospitable place than it is today, home to numerous freshwater lakes. As the region became drier, people might have moved east into Asia or returned to Africa.

Of course, when the only remains at an archaeological site are tools, it’s hard to say with absolute certainty who made them. The researchers working in Oman noted that the tools they found in Arabia match the technology of modern humans found in eastern Africa about 128,000 years ago. The team made the case that the tool makers on either side of the Red Sea belonged to the same cultural group—and therefore the same species. But as anthropologists discover more species, such as the Hobbit or the Denisovans, that lived alongside modern humans outside of Africa up until a few tens of thousands of years ago, it becomes harder to say stone tools alone indicate the presence of Homo sapiens.

DNA: Genetic data can help fill in the holes in the human migration story that fossils and artifacts can’t address. Anthropologists collect DNA samples from different ethnic groups around the world. Next, they count up the genetic differences caused by mutations in certain sections of the genome. Groups that are more closely related will have fewer genetic differences, which implies they split off more recently form each other than they did with more distantly related groups. Scientists calculate when in the past different groups diverged from each other by adding up all of the genetic differences between two groups and then estimating how often genetic mutations occurred. Such analyses not only give a sense of when different parts of the world were first inhabited, but they can also reveal more intricate patterns of movement. For example, genetic data suggest North America was colonized by three separate waves of people leaving Siberia across the Bering Strait.

Genetic data are not foolproof, however. The estimated divergence times are only as accurate as the estimated mutation rate, which scientists still debate. In the early days of DNA studies, scientists used either mitochondrial DNA, passed down only by the mother, or the Y chromosome, inherited only from father to son. Neither of these types of DNA presented the full picture of what people were doing in the past, as mitochondrial DNA only tracks maternal lineages while the Y chromosome only follows paternal lines. Today, whole genome sequencing is beginning to allow researchers to trace entire populations.

Languages: Anthropologists use languages in methods analogous to studying DNA; they look for patterns of similarities, or differences, in vocabularies or other aspects of language. Earlier this year, researchers compared different languages within the Indo-European language family to determine where these languages arose. After assessing the relationship between the languages, the researchers considered the geographic ranges where those languages are currently spoken. They concluded that the Indo-European language family originated in what is today Turkey and then spread west into Europe and east into southern Asia as people moved into these areas. But such linguistic analyses may only track relatively recent migration patterns. For example, H. Craig Melchert, a linguist at the University of California, Los Angeles, told Science News that the Indo-European languages can only be traced back about 7,000 years.