Thursday, 20 September 2007

Postcranial evidence from early Homo from Dmanisi, Georgia

Lordkipanidze et al (2007)

The Plio-Pleistocene site of Dmanisi, Georgia, has yielded a rich fossil and archaeological record documenting an early presence of the genus Homo outside Africa. Although the craniomandibular morphology of early Homo is well known as a result of finds from Dmanisi and African localities, data about its postcranial morphology are still relatively scarce. Here we describe newly excavated postcranial material from Dmanisi comprising a partial skeleton of an adolescent individual, associated with skull D2700/D2735, and the remains from three adult individuals. This material shows that the postcranial anatomy of the Dmanisi hominins has a surprising mosaic of primitive and derived features. The primitive features include a small body size, a low encephalization quotient and absence of humeral torsion; the derived features include modern-human-like body proportions and lower limb morphology indicative of the capability for long-distance travel. Thus, the earliest known hominins to have lived outside of Africa in the temperate zones of Eurasia did not yet display the full set of derived skeletal features.

Source:Nature 449, 305-310 (20 September 2007)
Treasure trove of Homo erectus foundDozens of fossils reveal four primative humans.
article by Rex Dalton

A trove of the oldest human skeletal bones outside Africa is reported in Nature this week — a find that will help researchers to improve their understanding of the biology of the 1.8-million-year-old hominins. The work, led by researchers from the Georgian National Museum in Tbilisi, describes three-dozen fossils from the skeletons of four primitive Homo erectus individuals found in recent years at Dmanisi in Georgia, central Asia. H. erectus is thought to have migrated across Asia after coming out of Africa, where the oldest relative of man is traced to nearly 7 million years ago. H. erectus fossils have been found from Africa across Asia as far as Indonesia. Typically there are only a few scattered fossils at one location. A single site with so many bones from so many individuals is rare. And they date back to very soon after H. erectus's exodus from Africa."Dmanisi is a real gift, because nothing in the world exists like this for this time," says lead author David Lordkipanidze."The really important point is you have multiple individuals from the same time and location," adds Tim White, a palaeoanthropologist at the University of California, Berkeley, who was not involved in the work. Together the specimens — three adults and an adolescent — present a much better picture of what the species was like as a whole than would a single skeleton. With one individual, experts note, it is difficult to determine whether a feature such as leg length is typical of the entire species or just characteristic of that one individual. With four skeletons, you start to have a data set that you can reasonably compare with modern humans, says Alan Walker, a palaeoanthropologist at Pennsylvania State University in University Park.Researchers are now attempting to link these fossils to three skulls, a cranium and a mandible all found previously in the same dig site.Size smallThe Dmanisi site — which continues to yield fossils annually — was near a lava flow where the primitive humans are thought to have scavenged carcasses for meat. But H. erectus then became a victim of carnivores, with their collective bones marked by animal teeth and found in a lair-like deposit.Lordkipanidze and his colleagues note that the skeletal fossils of shoulder, arm, spine and leg show that the individuals were small (about 50 kilograms on a frame of some 150 centimetres tall), had modern-human body-limb proportions, and legs capable of long-distance travel.This reflects variation expected in the species, notes anatomist Owen Lovejoy of Kent State University in Ohio. It is known that H. erectus living in colder climates had shorter limbs compared with those from the hotter environs of Africa.Even though this sample provides a good look at H. erectus in this time and place, experts caution against drawing broad conclusions about H. erectus. As more fossils are reported in the near future, as is expected, the growing Dmanisi collection will allow researchers to describe our relatives more definitively.
Source:Nature doi:10.1038/news070917-6

Monday, 17 September 2007

These Legs were Made for Fighting;Human Ancestors had Short Legs for Combat, not Just Climbing

Ape-like human ancestors known as australopiths maintained short legs for 2 million years because a squat physique and stance helped the males fight over access to females, a University of Utah study concludes.
"The old argument was that they retained short legs to help them climb trees that still were an important part of their habitat," says David Carrier, a professor of biology. "My argument is that they retained short legs because short legs helped them fight."
The study analyzed leg lengths and indicators of aggression in nine primate species, including human aborigines. It is in the March issue of the journal Evolution.
Creatures in the genus Australopithecus – immediate predecessors of the human genus Homo – had heights of about 3 feet 9 inches for females and 4 feet 6 inches for males. They lived from 4 million to 2 million years ago.
"For that entire period, they had relatively short legs – longer than chimps' legs but shorter than the legs of humans that came later," Carrier says.
"So the question is, why did australopiths retain short legs for 2 million years? Among experts on primates, the climbing hypothesis is the explanation. Mechanically, it makes sense. If you are walking on a branch high above the ground, stability is important because if you fall and you're big, you are going to die. Short legs would lower your center of mass and make you more stable."
Yet Carrier says his research suggests short legs helped australopiths fight because "with short legs, your center of mass is closer to the ground. It's going to make you more stable so that you can't be knocked off your feet as easily. And with short legs, you have greater leverage as you grapple with your opponent."
While Carrier says his aggression hypothesis does not rule out the possibility that short legs aided climbing, but "evidence is poor because the apes that have the shortest legs for their body size spend the least time in trees – male gorillas and orangutans."
He also notes that short legs must have made it harder for australopiths "to bridge gaps between possible sites of support when climbing and traveling through the canopy."
Nevertheless, he writes, "The two hypotheses for the evolution of relatively short legs in larger primates, specialization for climbing and specialization for aggression, are not mutually exclusive. Indeed, selection for climbing performance may result in the evolution of a body configuration that improves fighting performance and vice versa."
Great Apes' Short Legs Provide Evidence for Australopith Aggression
All modern great apes – humans, chimps, orangutans, gorillas and bonobos – engage in at least some aggression as males compete for females, Carrier says.
Carrier set out to find how aggression related to leg length. He compared Australian aborigines with eight primate species: gorillas, chimpanzees, bonobos, orangutans, black gibbons, siamang gibbons, olive baboons and dwarf guenon monkeys. Carrier used data on aborigines because they are a relatively natural population.
For the aborigines and each primate species, Carrier used the scientific literature to obtain typical hindlimb lengths and data on two physical features that previously have been shown to correlate with male-male competition and aggressiveness in primates:
The weight difference between males and females in a species. Earlier studies found males fight more in species with larger male-female body size ratios.
The male-female difference in the length of canine teeth, which are next to the incisors and are used for biting during fights.
Carrier used male-female body size ratios and canine tooth size ratios as numerical indicators for aggressiveness because field studies of primates have used varying criteria to rate aggression. He says it would be like having a different set of judges for each competitor in subjective Olympic events like diving or ice dancing.
The study found that hindlimb length correlated inversely with both indicators of aggressiveness: Primate species with greater male-female differences in body weight and length of the canine teeth had shorter legs, and thus display more male-male combat.
There was no correlation between arm length and the indicators of aggression. Carrier says arms are used for fighting, but "for other things as well: climbing, handling food, grooming. Thus, arm length is not related to aggression in any simple way."
Verifying the Findings
Carrier conducted various statistical analyses to verify his findings. First, he corrected for each species' limb lengths relative to their body size. Primates with larger body sizes tend to have shorter legs, humans excepted. Without taking that into account, the correlation between body size and aggression indicators might be false.
Another analysis corrected for the fact different primate species are related. For example, if three closely related species all have short legs, it might be due to the relationship – an ancestor with short legs – and not aggression.
Even with the corrections, short legs still correlated significantly with the two indicators of aggressiveness.
The study also found that females in each primate species except humans have relatively longer legs than males. "If it is mainly the males that need to be adapted for fighting, then you'd expect them to have shorter legs for their body size," Carrier says.
He notes there are exceptions to that rule. Bonobos have shorter legs than chimps, yet they are less aggressive. Carrier says the correlation between short legs and aggression may be imperfect because legs are used for many other purposes than fighting.
Humans "are a special case" and are not less aggressive because they have longer legs, Carrier says. There is a physical tradeoff between aggression and economical walking and running. Short, squat australopiths were strong and able to stand their ground when shoved, but their short legs made them ill-suited for distance running. Slender, long-legged humans excel at running. Yet, they also excel at fighting. In a 2004 study, Carrier made a case that australopiths evolved into lithe, long-legged early humans only when they learned to make weapons and fight with them.
Now he argues that even though australopiths walked upright on the ground, the reason they retained short legs for 2 million years was not so much that they spent time in trees, but "the same thing that selected for short legs in the other great apes: male-male aggression and competition over access to reproductively active females."
In other words, shorter legs increased the odds of victory when males fought over access to females – access that meant passing their genetic traits to offspring.
Yet, "we don't really know how aggressive australopiths were," Carrier says. "If they were more aggressive than modern humans, they were exceptionally nasty animals."
Why Should We Care that Australopiths Were Short and Nasty?
"Given the aggressive behavior of modern humans and apes, we should not be surprised to find fossil evidence of aggressive behavior in the ancestors of modern humans," Carrier says. "This is important because we have a real problem with violence in modern society. Part of the problem is that we don’t recognize we are relatively violent animals. Many people argue we are not violent. But we are violent. If we want to prevent future violence we have to understand why we are violent."
"To some extent, our evolutionary past may help us to understand the circumstances in which humans behave violently," he adds. "There are a number of independent lines of evidence suggesting that much of human violence is related to male-male competition, and this study is consistent with that."
Nevertheless, male-male competition doesn’t fully explain human violence, Carrier says, noting other factors such as hunting, competing with other species, defending territory and other resources, and feeding and protecting offspring.

Source: University of Utah,2007

Study identifies energy efficiency as reason for evolution of upright walking

A new study provides support for the hypothesis that walking on two legs, or bipedalism, evolved because it used less energy than quadrupedal knucklewalking.
David Raichlen, an assistant professor of anthropology at The University of Arizona, conducted the study with Michael Sockol from the University of California, Davis, who was the lead author of the paper, and Herman Pontzer from Washington University in St. Louis.
Raichlen and his colleagues will publish the article, "Chimpanzee locomotor energetics and the origin of human bipedalism" in the online early edition of the Proceedings of the National Academy of Sciences (PNAS) during the week of July 16. The print issue will be published on July 24.
Bipedalism marks a critical divergence between humans and other apes and is considered a defining characteristic of human ancestors. It has been hypothesized that the reduced energy cost of walking upright would have provided evolutionary advantages by decreasing the cost of foraging.
"For decades now researchers have debated the role of energetics and the evolution of bipedalism," said Raichlen. "The big problem in the study of bipedalism was that there was little data out there."
The researches collected metabolic, kinematic and kenetic data from five chimpanzees and four adult humans walking on a treadmill. The chimpanzees were trained to walk quadrupedally and bipedally on the treadmill.
Humans walking on two legs only used one-quarter of the energy that chimpanzees who knuckle-walked on four legs did. On average, the chimpanzees used the same amount of energy using two legs as they did when they used four legs. However, there was variability among chimpanzees in how much energy they used, and this difference corresponded to their different gaits and anatomy.
"We were able to tie the energetic cost in chimps to their anatomy," said Raichlen. "We were able to show exactly why certain individuals were able to walk bipedally more cheaply than others, and we did that with biomechanical modeling."
The biomechanical modeling revealed that more energy is used with shorter steps or more active muscle mass. Indeed, the chimpanzee with the longest stride was the most efficient walking upright.
"What those results allowed us to do was to look at the fossil record and see whether fossil hominins show adaptations that would have reduced bipedal energy expenditures," said Raichlen. "We and many others have found these adaptations [such as slight increases in hindlimb extension or length] in early hominins, which tells us that energetics played a pretty large role in the evolution of bipedalism."

Source: University of Arizona Communications; 2007

Was ability to run early man's Achilles heel?

The earliest humans almost certainly walked upright on two legs but may have struggled to run at even half the speed of modern man, new research suggests.
The University of Manchester study- presented to the BA (British Association for the Advancement of Science) Festival of Science in Yorktoday (Tuesday)- proposes that if early humans lacked an Achilles tendon, as modern chimps and gorillas do, then their ability to run would have been severely compromised.
"Our research supports the belief that the earliest humans used efficient bipedal walking rather than chimp-like 'Groucho' walking," said Dr Bill Sellers, who led the research in the University's Faculty of Life Sciences.
"But if, as seems likely, early humans lacked an Achilles tendon then whilst their ability to walk would be largely unaffected our work suggests running effectiveness would be greatly reduced with top speeds halved and energy costs more than doubled.
"Efficient running would have been essential to allow our ancestors to move from a largely herbivorous diet to the much more familiar hunting activities associated with later humans. What we need to discover now is when in our evolution did we develop an Achilles tendon as knowing this will help unravel the mystery of our origins."
Dr Sellers, who recently published research on the running speeds of five meat-eating dinosaurs, used the same computer software to generate a humanoid bipedal computer model using data from a hominid fossil skeleton called 'Lucy' and hominid footprints preserved in ash at Laetoli in Tanzania.
"The skeletons and footprints from some of the earliest members of the human lineage- the early hominids- provide the best clues we have to how we progressed down the pathway to modern human walking and running," said Dr Sellers.
"We have borrowed techniques from other scientific disciplines - robotics, computer science and biomechanics - in an attempt to 'reverse engineer' fossil skeletons; we use what we know about skeletons and the muscles to build a computer model of the fossil species we are interested in.
"This model is a virtual robot where we can activate muscles and get it to move its legs in a physically realistic fashion; the tricky bit is getting it to actually walk or run without falling over.
"However, if we use big enough computers and let the model fall over enough times it is possible for the simulation to learn which muscles to fire and when in order to get the model to walk properly. Even better we can ask the computer to find ways of minimising fuel cost and maximising top speed since that is what we think animals have to do."
Dr Sellers initially looked at walking and his models suggested that even as early as 3.5 million years ago our human ancestors were able to walk as efficiently as modern humans. His research also showed that they preferred to walk a little slower than we do but only because they were much smaller and had quite short legs.
The team also used the computer model to look at particular parts of the human locomotion system, including the Achilles tendon, which they showed acts like a big spring to store energy during running; when the tendon was removed from the model the top running speed was greatly reduced.
"We have only just started to look at running and so there are still plenty of questions to answer," said Dr Sellers. "But whilst these very early fossils could walk well, our initial findings suggest that efficient running came about quite a bit later in the fossil record.
"How we evolved from our common ancestor with chimpanzees six million years ago is a fundamental question. Walking upright seems to be the very first thing that distinguishes our ancestors from other apes, so finding out about this should help us map the evolutionary pathway to modern humans.
"The next really interesting question is to look in more detail at running. It has been suggested that our ability to run for long distances took a lot longer to evolve than our ability to walk and that this is a defining feature of our very close relatives in our genus. Our techniques should let us get to the bottom of this question because it will let us measure the running abilities of our fossil ancestors directly."

Source:Manchester University; Faculty of Life Science. 17 th September 2007

Tuesday, 11 September 2007

Our upright walking started in the trees

Our ancient tree-dwelling ancestors stood upright on two legs – a trait modern humans have retained, while other great apes have evolved four-legged knuckle-walking, researchers say.
It was one of the characteristics that was supposed to define the ancestral human line from our great ape cousins: We walk upright, while chimps and gorillas walk on four legs, using their knuckles.
Now it seems that we did not evolve from knuckle-walkers – bipedalism apparently arose far earlier in evolutionary history, when our ancestors were still in the trees.
Several scenarios have been proposed to explain why we came to walk on two legs, from the idea that it allowed our ancestors to
feed more efficiently and carry infants, to the idea that the posture reduced our exposure to sunlight and so gave us longer to forage.
Canopy skills
Susannah Thorpe, of the University of Birmingham, UK, and colleagues have made extensive observations of the most arboreal of the modern great apes, the orang-utan, and come to another conclusion, that bipedalism evolved to help us move about the forest canopy.
"The orang-utan is the only great ape still living in its ancestral habitat," says Thorpe, making them good models for understanding the selection pressures on ancestral apes.
Thorpe spent a year recording orang-utan behaviour in the Gunung Leuser National Park in Sumatra, Indonesia, and from nearly 3000 observations of locomotion, the team concluded that the apes were more likely to walk on two legs - using their hands to guide them - when they are on the thinnest branches, less than 4 centimetres in diameter.
On medium-sized branches - those greater than 4 cm but less than 20 cm diameter - the apes tended to walk bipedally, but used their arms to support their weight by swinging or hanging.
Only on the largest branches, with a diameter greater than 20 cm, did the animals walk on all fours.
Fruits of bipedalism
Since orang-utans are fruit eaters, and fruit is more likely to be found on the ends of thinner branches, the ability to walk out along them is advantageous. Since they also spend most of their time in the trees, the ability to move over thinner branches helps when it comes to crossing from tree to tree and traversing the canopy, says Thorpe.
When walking bipedally, orang-utans extended their legs at the knee and hip to give them a straighter posture, in contrast to what happens when chimps try to walk on two legs. Chimps are forced to waddle with bent knees and their torso bent over at the hip. When humans run on springy surfaces they also keep their legs straight, like orang-utans, Thorpe points out.
"Walking upright and balancing themselves by holding branches with their hands is an effective way of moving on smaller branches," says Robin Crompton of the University of Liverpool, UK, who was also involved in the study.
"It helps to explain how early human ancestors learnt to walk upright while living in the trees, and how they would have used this way of moving when they left the trees for a life on the ground."
Ground foragers
So, rather than evolving to walk on two feet after scrabbling around on the floor on all fours, the theory suggests our ancestors already had the rudimentary means of walking on two feet before they even left the trees.
When the ancestors of chimps and gorillas left the trees, however, they needed to maintain the ability to climb tree trunks. This need for tree-climbing strength and anatomy guided their evolution at the expense of more efficient terrestrial movement, and therefore led to knuckle-walking, says Crompton.
Orang-utans are the most distant of our relatives among the great apes, followed by gorillas, and then bonobos and chimpanzees. The ancestors of the latter two species split from the human line around 6 million years ago; the orang-utan ancestor split from the human ancestor around 10 million years ago.
Thorpe and colleagues suggest that at sometime in the Miocene epoch - 24 to 5 million years ago - the increased gaps in the forest canopy that came about as a result of climate fluctuations had a profound effect on our ape ancestors.
Some of them - the ancestors of chimps and gorillas - specialised on climbing high into the canopy and crossing the gaps between trees by knuckle walking. Others - the ancestors of humans - retained their ability to walk on two legs, and specialised on collecting food from smaller trees and the ground.
Plausible mechanism
The idea of an arboreal origin for human bipedalism has been proposed before, says Chris Stringer, a palaeontologist at the Natural History Museum, London. "Nevertheless, this is the best observational data on the importance of hand-assisted bipedalism to orangs, and its possible implications for the evolution of human bipedalism."
Since all the sites which have yielded fossil evidence of our earliest ancestors were forested or wooded, rather than open, Stringer says, "arboreal bipedalism is certainly a very plausible mechanism for the origins of walking upright."
Paul O’Higgins, of the Functional Morphology and Evolution unit at the University of York, UK, says the finding makes it more difficult to find a feature unique to the human ancestral line. "If extended hip and knee bipedalism did indeed arise in the distant past, this makes the task of identifying possible ancestors of the human line much more difficult," he says.
There has been tantalising fossil evidence suggesting an early origin for bipedalism, says Crompton. "And the orang-utan is the only ape with a knee joint similar to that of humans."
Despite this, the idea that all apes at one stage had the potential to walk on two legs and that from this starting point some evolved to knuckle walk and some evolved bipedalism has been resisted. "Perhaps we’ve been too focused on the African apes," says Thorpe. "The trend has been to look at them to explain human evolution."

31 May 2007 news service
Rowan Hooper
Journal reference: Science (DOI: 10.1126/science.1140799)

Ancient gene kit came in handy for limbs

The master genes controlling development in the primitive animal known as the paddlefish turn out to be unexpectedly similar to those controlling the development of limbs in land animals. Rather than evolving a new set of control genes for their limbs, it seems that our amphibian ancestors adapted the genes their own ancestors used to develop fins.
The paddlefish Polyodon spathula is often referred to as a "living fossil", an organism that is similar to no known species apart from fossils. Most previous genetic work on fish has been done on the more highly evolved zebrafish. This appeared to show that the Hox family of control genes in land animals and fish were different, implying land animals had evolved new genes to control growth of hands and feet.
Hox genes control the alignment and polarisation of body structures in all animals, separating head from tail. In fish they arrange the structures of fins, and in land animals the structure of limbs. Zebrafish develop their fins in a single stage, in which Hox genes produce parallel stripes that underlie fin structures. Mice and chickens have a second phase of development in which Hox genes turn on only in the regions that become a hand or foot.
"The logical explanation was that since fish don't have hands, they don't have a second stage of Hox gene development, so [addition of] the second stage should correlate with evolution of the hand," says Marcus Davis of the University of Chicago. But zebrafish are highly evolved, so he wondered if they had lost the ancestral form of fin development.
Davis and colleagues looked at Hox genes in the paddlefish, a primitive relative of the sturgeon, because it is relatively unevolved. They found that the little arm fins of paddlefish develop in two phases, implying that the second phase of Hox gene expression had evolved long before arms and legs, but was lost in zebrafish (Nature, vol 447, p 473).
That ancient set of genes played a key role in helping vertebrates crawl onto land. Tiktaalik, the fish with feet discovered last year (New Scientist, 8 April 2006, p 14), "already had the toolkit needed to modify the part of the limbs furthest out", Davis says. As ancestral amphibians moved onto land, they used their existing genetic tools to adapt their limbs.
“As amphibians moved onto land they used their existing genetic tools to adapt their limbs to the new environment”
"Here's something we thought was invented from scratch, but it was there in a deep ancestor of tetrapods," says Sean Carroll, a developmental biologist at the University of Wisconsin, Madison.

From issue 2605 of New Scientist magazine, 23 May 2007, page 18

Saturday, 1 September 2007

The evolution of human running: Effects of changes in lower-limb length on locomotor economy

Previous studies have differed in expectations about whether long limbs should increase or decrease the energetic cost of locomotion. It has recently been shown that relatively longer lower limbs (relative to body mass) reduce the energetic cost of human walking. Here we report on whether a relationship exists between limb length and cost of human running. Subjects whose measured lower-limb lengths were relatively long or short for their mass (as judged by deviations from predicted values based on a regression of lower-limb length on body mass) were selected. Eighteen human subjects rested in a seated position and ran on a treadmill at 2.68 m s−1 while their expired gases were collected and analyzed; stride length was determined from videotapes. We found significant negative relationships between relative lower-limb length and two measures of cost. The partial correlation between net cost of transport and lower-limb length controlling for body mass was r = −0.69 (p = 0.002). The partial correlation between the gross cost of locomotion at 2.68 m s−1 and lower-limb length controlling for body mass was r = −0.61 (p = 0.009). Thus, subjects with relatively longer lower limbs tend to have lower locomotor costs than those with relatively shorter lower limbs, similar to the results found for human walking. Contrary to general expectation, a linear relationship between stride length and lower-limb length was not found.

Steudel-Numbers, K. L. Weaver, T. D. Wall-Scheffler,C. M. (2007)Department of Zoology, University of Wisconsin, Madison, WI 53706, USA