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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.

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.

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.

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.

New research suggests the timing of human gestation is not a compromise between the size of a woman’s hips and the size of a baby’s head. Instead, it’s determined by a woman’s energy limits. Image: xopherlance/Flickr

Have you ever wondered why women stay pregnant for nine months? For decades, anthropologists have explained the timing of human gestation and birth as a balance between two constraints: the size of a women’s hips and the size of a newborn’s brain. But new research says that’s not the case. Instead, the timing of childbirth occurs when women’s bodies can no longer keep up with the energy demands of pregnancy. That happens at around nine months, Holly Dunsworth of the University of Rhode Island and colleagues report online August 27 in the Proceedings of the National Academy of Sciences.

The traditional explanation of gestation length is known as the obstetric dilemma. The hypothesis suggests that the width of the pelvis, and thus the width of the birth canal, is limited by the demands of efficient upright walking. But as brain size expanded over hominid evolution, heads got bigger. To make sure a baby’s head could fit through the birth canal, gestation decreased and babies were born at an earlier stage of development; today, newborns enter the world with the least developed brain of all primates at less than 30 percent adult size.

Dunsworth and her colleagues wanted to see if they could find any actual evidence to support the obstetric dilemma. First, they considered gestation length. Traditionally, human gestation has been considered short when looking at how much additional growth the brain needs to reach adult size. But such a measure is unfair when compared to other primates since humans have abnormally large brains, the researchers say. Instead, Dunsworth’s team compared gestation length to maternal body size and found humans actually have relatively long pregnancies—37 days longer than would be expected for a typical primate our size. Our gestation is also relatively extended compared with chimpanzees or gorillas, suggesting pregnancies got longer, not shorter, in hominids.

The team also looked for evidence that widening the pelvis to accommodate bigger brained babies would make walking less efficient. Researchers have assumed that broadening the hips would increase the force needed by hip muscles to walk and run, thus making locomotion less energy efficient. But one recent study shows the dimensions of the hips don’t actually affect the muscle’s required force, calling into question the long-held belief that wider hips would interfere with women’s walking. Furthermore, the team calculated how much wider the hips would have to be if humans were born with the same brain development as chimps (40 percent adult size). All that would be needed is a three-centimeter increase. Women’s hips already vary by three or more centimeters, the researchers say, suggesting that hip size really doesn’t limit gestation.

Instead, gestation is determined by energy. Studies of mammals show that during pregnancy females reach their species’ “metabolic ceiling,” the upper limit of the amount of energy they can expend. In humans, the metabolic ceiling is 2 to 2.5 times the baseline amount of energy needed during rest. Dunsworth and her colleagues say women reach that limit by their sixth month of pregnancy. Then at nine months, the energy demands of a fetus go beyond this metabolic threshold. “Extending gestation even by a month would likely require metabolic investment beyond the mother’s capacity,” the team writes.

But even though hip size doesn’t appear to limit the size of a baby’s head, women around the world often have trouble delivering babies because of the tight fit of the head going through the birth canal. One possible explanation is that childbirth has only become problematic recently in human evolution. Changes in diet that have led to increased energy consumption may be allowing women to produce bigger babies, and natural selection hasn’t had enough time to broaden the hips. Figuring out why modern childbirth is so difficult, and dangerous, is an area that needs further research.