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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.

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.

Human running is less efficient than the running of a typical mammal with the same body mass, a new study finds. Image: tcd123usa/Flickr

Why hominids evolved upright walking is one of the biggest questions in human evolution. One school of thought suggests that bipedalism was the most energetically efficient way for our ancestors to travel as grasslands expanded and forests shrank across Africa some five million to seven million years ago. A new study in the Journal of Human Evolution challenges that claim, concluding that the efficiency of human walking and running is not so different from other mammals.

Physiologists Lewis Halsey of the University of Roehampton in England and Craig White of the University of Queensland in Australia compared the efficiency of human locomotion to that of 80 species of mammals, including monkeys, rodents, horses, bears and elephants. For each species, Halsey and White computed the “net cost of transport,” a figure that considers an animal’s metabolic rate (measured in oxygen consumption), given its speed, while traveling one meter. Next, they created an equation that predicts a mammal’s net cost of transport based on its body mass.

The researchers found that a typical mammal weighing 140 pounds (the average weight for humans) has a net cost of transport of 10.03 milliliters of oxygen per meter while running. Human running on average requires 12.77 milliliters of oxygen per meter—27 percent more than the researchers’ calculation. In contrast, human walking is 25 percent more efficient than the average, same-sized mammal’s walking. The team also estimated that the roughly three-million-year-old Australopithecus afarensis‘ walking was 26 to 37 percent more efficient than the average mammal’s, depending on the estimated weight of the chimp-sized hominid.

Although modern humans and A. afarensis are more efficient walkers than the average mammal, Halsey and White argue that neither species is exceptional. When looking at all of the data points, both hominids fall within the 95 percent prediction interval for mammals. Statistically speaking, that’s the range you’d expect 95 percent of predicted mammalian net transport costs to fall within on average. In other words, modern humans and A. afarensis fall within the normal realm of variation for mammals. There’s nothing special about the energetics of their walking, Halsey and White conclude.

To evaluate whether energy efficiency played a role in the evolution of upright walking, Halsey and White note that hominids should be compared to their closest relatives. For example, if human walking is more efficient than chimpanzee walking than you would expect based on chance alone, then it lends support to the energy-efficiency explanation. But that’s not what the researchers found. In fact, the energetic differences between humans and  chimpanzees are smaller than the differences between very closely related species that share the same type of locomotion, such as red deer versus reindeer or African dogs versus Arctic foxes. In some cases, even different species within the same genus, such as different types of chipmunks, have greater variation in their walking efficiencies than humans and chimps do. The researchers speculate that factors like climate and habitat might explain why such similar animals have such different locomotor costs.

This one study is unlikely to be the last word on the matter. I’m curious how the estimated energy efficiency of A. afarensis compares to chimpanzees, or even to modern humans, something the researchers didn’t examine. It would also be interesting to calculate the net transport cost for the 4.4-million-year-old Ardipithecus, the oldest hominid for which anthropologists have a complete skeleton. That seems like the crucial test of whether energy efficiency played some kind of role in the evolution of bipedalism.

KNM-ER 1470, a Homo rudolfensis specimen, is one of the fossils you can examine at African Fossils. Image: José-Manuel Benito Álvarez/Wikicommons

Last fall, I offered my picks for the best places to see hominid bones online. I thought it was time to share some more great human evolution Web sites that I’ve discovered.

Fossilized.org: This site is filled with a ton of information on the different places where hominid fossils and stone tools have been found. The homepage is a world map locating the archaeological sites. Next to the map is a list of some of these places; clicking on a name brings up a satellite image of the area and more information on the location’s significance. The site also  includes a timeline of important events in the history of paleoanthropology, a geologic timescale and a list of all the hominid species, including the year the species was first recognized. Anthropologist William Henry Gilbert of California State University, East Bay made the Web site.

African Fossils: A virtual anthropology lab that feels like a video game, this site is the brainchild of Louise Leakey, Louis and Mary Leakey’s granddaughter. It displays specimens from the collections of the National Museums of Kenya. Still a work in progress, the site lets you navigate through the lab and click on different objects to learn more about them. The best part is playing with the digital, 3-D hominid fossils and rotating them to see the specimens from different angles.

Ardipithecus Handbook: Brought to you by the Discovery Channel, this Web site is an interactive guide to the approximately four-million- to six-million-year-old genus, with a special emphasis on the famous skeleton named Ardi. The handbook offers background on Ethiopia’s Middle Awash, where Ardi and other hominids have been found—including an interactive map that locates and describes different hominid fossils discoveries—as well as a discussion of the genus’s place in the human family tree. The site also has an interactive Ardi skeleton that provides 3-D views of different bones.

Bones, Stones and Genes: The Origin of Modern Humans lecture series: The subject of the Howard Hughes Medical Institute’s 2011 Holiday Lectures was human evolution, and the institute has archived high-quality videos of these talks. The lectures are given by top anthropologists and are a great introduction to the science of human evolution. Paleoanthropologist Tim White of the University of California, Berkeley discusses his Middle Awash field site, where his team found Ardi and the 160,000-year-old Herto fossils, some of the earliest remains of Homo sapiens. Genetecist Sarah Tishkoff of the University of Pennsylvania offers a tutorial in human genetics. And archaeologist John Shea of Stony Brook University describes the earliest stone tools and the ways in which scientists study them. His talk also includes tool-making demonstrations.

A trio of upright walkers: Lucy (middle) and Australopithecus sediba (left and right). Image: Compiled by Peter Schmid courtesy of Lee R. Berger, University of the Witwatersrand/Wikicommons

Welcome to Hominid Hunting’s new series “Becoming Human,” which will periodically examine the evolution of the major traits and behaviors that define humans, such as big brains, language, technology and art. Today, we look at the most fundamental human characteristic: walking upright. 

Walking upright on two legs is the trait that defines the hominid lineage: Bipedalism separated the first hominids from the rest of the four-legged apes. It took a while for anthropologists to realize this. At the turn of the 20th century, scientists thought that big brains made hominids unique. This was a reasonable conclusion since the only known hominid fossils were of brainy species–Neanderthals and Homo erectus.

That thinking began to change in the 1920s when anatomist Raymond Dart discovered the skull known as the Taung Child in South Africa. Taung Child had a small brain, and many researchers thought the approximately three-million-year-old Taung was merely an ape. But one feature stood out as being human-like. The foramen magnum, the hole through which the spinal cord leaves the head, was positioned further forward under the skull than an ape’s, indicating that Taung held its head erect and therefore likely walked upright. In the 1930s and 1940s, further fossil discoveries of bipedal apes that predated Neanderthals and H. erectus (collectively called australopithecines) helped convince anthropologists that walking upright came before big brains in the evolution of humans. This was demonstrated most impressively in 1974 with the finding of Lucy, a nearly complete australopithecine skeleton. Although Lucy was small, she had the anatomy of a biped, including a broad pelvis and thigh bones that angled in toward the knees, which brings the feet in line with the body’s center of gravity and creates stability while walking.

In more recent decades, anthropologists have determined that bipedalism has very ancient roots. In 2001, a group of French paleoanthropologists unearthed the seven-million-year-old Sahelanthropus tchadensis in Chad. Known only from a skull and teeth, Sahelanthropus‘ status as an upright walker is based solely on the placement of its foramen magnum, and many anthropologists remain skeptical about the species’ form of locomotion. In 2000, paleoanthropologists working in Kenya found the teeth and two thigh bones of the six-million-year-old Orrorin tugenensis. The shape of the thigh bones confirms Orrorin was bipedal. The earliest hominid with the most extensive evidence for bipedalism is the 4.4-million-year-old Ardipithecus ramidus. In 2009, researchers announced the results of more than 15 years of analysis of the species and introduced the world to a nearly complete skeleton called Ardi.

Although the earliest hominids were capable of upright walking, they probably didn’t get around exactly as we do today. They retained primitive features—such as long, curved fingers and toes as well as longer arms and shorter legs—that indicate they spent time in trees. It’s not until the emergence of H. erectus 1.89 million years ago that hominids grew tall, evolved long legs and became completely terrestrial creatures.

While the timeline of the evolution of upright walking is well understood, why hominids took their first bipedal steps is not. In 1871, Charles Darwin offered an explanation in his book The Descent of Man: Hominids needed to walk on two legs to free up their hands. He wrote that “…the hands and arms could hardly have become perfect enough to have manufactured weapons, or to have hurled stones and spears with a true aim, as long as they were habitually used for locomotion.” One problem with this idea is that the earliest stone tools don’t show up in the archaeological record until roughly 2.5 million years ago, about 4.5 million years after bipedalism’s origin.

But after the unveiling of Ardi in 2009, anthropologist C. Owen Lovejoy of Kent State University revived Darwin’s explanation by tying bipedalism to the origin of monogamy. I wrote about Lovejoy’s hypothesis for EARTH magazine in 2010. Lovejoy begins by noting that Ardi’s discoverers say the species lived in a forest. As climatic changes made African forests more seasonal and variable environments, it would have become harder and more time-consuming for individuals to find food. This would have been especially difficult for females raising offspring. At this point, Lovejoy suggests, a mutually beneficial arrangement evolved: Males gathered food for females and their young and in return females mated exclusively with their providers. To be successful providers, males needed their arms and hands free to carry food, and thus bipedalism evolved. This scenario, as with all bipedalism hypotheses, is really hard to test. But earlier this year, researchers offered some support when they found that chimpanzees tend to walk bipedally when carrying rare or valuable foods.

Another theory considers the efficiency of upright walking. In the 1980s, Peter Rodman and Henry McHenry, both at the University of California, Davis, suggested that hominids evolved to walk upright in response to climate change. As forests shrank, hominid ancestors found themselves descending from the trees to walk across stretches of grassland that separated forest patches. The most energetically efficient way to walk on the ground was bipedally, Rodman and McHenry argued. (Full disclosure: Rodman was my graduate school advisor.) In 2007, researchers studying chimpanzees on treadmills determined that the chimps required 75 percent more energy while walking than two-legged humans, providing some evidence that bipedalism has advantages.

Numerous other explanations for bipedalism have been outright rejected, such as the idea that our ancestors needed to stand up to see over tall grass or to minimize the amount of the body exposed to the sun in a treeless savannah. Both ideas were debunked by the fact that the first hominids lived in at least partially wooded habitats.

Although difficult to study, the question of why bipedalism evolved might come closer to an answer if paleoanthropologists dig up more fossils of the earliest hominids that lived seven million to six million years ago. Who knows how many species of bipedal apes they’ll find. But each new discovery has the potential to fundamentally change how we understand the origins of one of our most distinctive traits.