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October 2009 Archives

by Michael Balter

blog_newskull Human evolution research is not for the faint-hearted. Hominin fossils are rare and hard to find. And more often than not, no sooner do anthropologists announce a big discovery than other researchers argue that they have it wrong. The next chapter in such a scenario unfolded last week, when scientists attending a meeting* at the Royal Society in London resurrected a debate about a single, crucial hominid specimen: a 3.5-million-year-old cranium named Kenyanthropus platyops—“the flat-faced man of Kenya” (shown at left).

Discovered in 1998 by a team including paleontologists Meave Leakey of the National Museums of Kenya in Nairobi and Fred Spoor of University College London, K. playtops suggests a greater degree of diversity in the human family tree than previously suspected: two species of hominids, not one, in the crucial period between about 4 million and 3 million years ago. That’s the time of Lucy, Australopithecus afarensis, whose lineage is thought by many to have given rise to our own genus, Homo.

But there was one nagging problem: The Kenyanthropus cranium, discovered west of Kenya’s Lake Turkana, was cracked and distorted, making it possible that some features that set it apart from A. afarensis—including its flat face and tall, vertically oriented cheek bones—could be artifacts. Paleoanthropologist Tim White of the University of California, Berkeley, argued in a 2003 Perspectives in Science that K. platyops probably fell within the range of variation among known A. afarensis fossils and might simply represent an “early Kenyan variant” of that species.

In London, Spoor responded to such arguments with a new, detailed study. He concluded from computed tomography scans that the upper jaw, or maxilla, had suffered much less distortion than the rest of the cranium. So he focused on that bone, correcting for the distortion present. He measured distances among several “landmarks” on the maxilla, including the point at which the cheekbone attaches to it, the extent of its forward projection, and the orientation of its tooth sockets. He used the same landmarks on A. afarensis specimens, the roughly 4-million-year-old A. anamensis, later australopithecines, and modern humans, chimps, and gorillas. Then he crunched the measurements in a computer-assisted analysis called principal component analysis to reveal the variability among the specimens. The result: Kenyanthropus fell cleanly outside the range of variation in all the other samples. “Species diversity existed at 3.5 million years ago, and this justifies assigning a new taxon,” Spoor concluded.

But White, who was present and has long argued that there is no evidence for more than one lineage of hominids at this time, wasn’t convinced. When the talk was thrown open for discussion, White took the microphone and began firing questions at Spoor about the degree of variation of the cheekbone position among specimens of A. afarensis and other hominin species. “We took that into account,” Spoor responded, “and I just showed you a graph” about it. “I didn’t ask you whether you took it into account; I asked you what it was,” White said. Spoor, clearly frustrated, told the audience that he had no vested interest in this debate. At that point, the session chair interrupted and invited everyone to break for coffee, but Spoor and White continued to debate between themselves for the next half-hour.

Spoor’s study, like the others presented at the meeting, will be published in the Philosophical Transactions of the Royal Society B: Biological Sciences in Spring 2010. When it is, the debate will no doubt continue.

*The First 4 Million Years of Human Evolution, London, 19-20 October 2009.

Photo credit: F. Spoor, copyright National Museums of Kenya

by Michael Balter

LONDON—When Tim White of the University of California, Berkeley, agreed to speak at a human origins meeting* at the Royal Society here, he sent no abstract and provided only a one-word title: “Ardipithecus.”

ardicover But that one word was enough to earn him a spot on the podium and a prominent role at the meeting. After 15 years of study, White and his colleagues have just published their massive, 108-page report on Ardipithecus ramidus, at 4.4 million years the oldest partial skeleton of a putative human ancestor (see the Science special issue of 2 October, cover at left). The 2-day meeting marked White’s first in-person presentation of Ardipithecus to a roomful of independent researchers; co-author anthropologist C. Owen Lovejoy of Kent State University in Ohio also made the trip. Their emphasis on Ardi’s non-apelike features, however, drew a tart response from some primatologists in the room.

The roughly 50-kilogram female, which the Ardipithecus team concludes walked upright although it also spent time in the trees, has a decidedly un-chimplike anatomy. White and colleagues therefore have asserted that living apes are not good models for understanding the last common ancestor (LCA) of humans and chimpanzees—a claim that has stung many primatologists. “I assumed … chimps might be helpful in tackling the challenges of human evolution,” said chimpanzee expert William McGrew of the University of Cambridge in the United Kingdom. But after Ardi, “all this chasing after chimpanzees was deemed to be irrelevant” to human evolution. McGrew, the first speaker, challenged a statement in one of the papers in Science that “no modern ape is a realistic proxy for characterizing early hominid evolution—whether social or locomotor.”

White, Lovejoy, and their colleagues base this conclusion on many features of Ardi’s postcranial anatomy. Although some researchers had assumed that the LCA had arms adapted for swinging in the trees and walked on its knuckles like chimps, A. ramidus lacks these and many other features possessed by modern apes such as chimps and gorillas. The team argues that Ardi’s anatomy suggests that its behavior and social structure also differed from that of modern chimps. For example, one of the papers, authored by Lovejoy, suggests that A. ramidus males were probably much less aggressive toward each other than chimp males are and that they pair-bonded with females.

30495239 McGrew told the attendees that although he agreed that “the LCA was not a chimpanzee,” the behavior of living chimps such as those at left can still inform hypotheses about the LCA’s behavior. McGrew listed the considerable evidence for chimp behaviors that possibly mirror those of extinct and modern humans, such as the use of complex tools, aiming and throwing objects, the construction of sleeping nests or platforms, and evidence for considerable spatial cognition such as the ability to remember the locations of thousands of trees in a forest. At least some of these behaviors, McGrew said, are shared by chimps and modern humans because they have deep evolutionary roots. “So to hypothetically credit the LCA with the ability of [chimps] is not unreasonable,” McGrew concluded. 

At the end of the day, when the meeting was thrown open for discussion among the roughly 200 attendees, White countered McGrew’s argument, pointing to what he saw as the dangers of using a chimp model for the LCA’s behavior. “If we try to model the LCA or even the earliest hominids based on living chimps, which have these adaptations to [swinging in the trees], to moving through that canopy so well and so quickly that they can take down a red colobus monkey, we could be very misled. Ardipithecus probably couldn’t do that, and the LCA probably couldn’t do that.” 

Yet White says he’s not trying to toss out all chimp research. “The fact that a few highly derived descendants managed to survive until today obviously enhances our appreciation of their evolution,” he told Science. “It also provides perspective on ours. The study and conservation of chimpanzees and bonobos are surely justifiable on their own merits, even though [early] hominid skeletal anatomy, behavior, and ecology do not match those seen now in these specialized persistent ape lineages.”

And Carol Ward, a paleoanthropologist from the University of Missouri School of Medicine in Columbia, points out that White and Lovejoy “aren’t saying that [studies of] chimps aren’t useful but that we didn’t evolve from chimps, so they are useful as referential models rather than actual representatives of what our ancestors were like.” Chris Stringer of the Natural History Museum in London, who co-organized the meeting with Alan Walker of Pennsylvania State University, University Park, also sees a role for chimp work in evolutionary studies. “There is such a lot of excellent and important data on ape behavior,”  Stringer told Science, “that we would be foolish to throw it out as providing potential models for the behavior of early hominins.”

*The First 4 Million Years of Human Evolution, London, 19-20 October 2009.

Photo credits (top to bottom): Cover; Jupiterimages

by Greg Miller

blog_octopus Cephalopods—octopuses, squid, and their relatives—ruled the seas in the Cambrian era, some 500 million years ago. But their world changed in a big way with the Cambrian Explosion, a rapid diversification of life on Earth that included the origin of fish. Suddenly, cephalopods had new opportunities—delicious fish!—and their first serious competition and potential predators. They had to get smart in a hurry.

So it’s no wonder then that modern cephalopods have the most complex brains of any invertebrates. An octopus brain (lower, right) has 50 to 75 lobes and at least as many neurons (about 100 million) as a mouse brain (lower, left). And that’s not counting the smaller "brains" in each arm and the still smaller "brains" (ganglia, technically) blog_2brainsassociated with each sucker.

All this neural circuitry gives octopuses exquisite control over their bodies, including some nifty tricks for evading predators, and it has even prompted speculation about cephalopod consciousness.

Although the octopus brain rivals the size and complexity of many vertebrate brains, its architecture differs dramatically. “Short of martians showing up and offering themselves up to science, cephalopods are the only example outside of vertebrates of how to build a complex, clever brain,” says neuroscientist Cliff Ragsdale of the University of Chicago in Illinois. For that reason, Ragsdale says, these creatures have much to teach us about brain evolution.

Ragsdale and his postdoctoral fellow Shuichi Shigeno are investigating whether the neural circuits that control movement, memory, and other functions in an octopus brain work the same way as do the analogous circuits in other animals. The British zoologist J. Z. Young did early work on octopuses in the 1960s, mapping out their brain anatomy and ablating individual lobes to investigate their contributions to behavior. But as of yet, Ragsdale says, no one has brought modern molecular methods to bear on these questions.

That’s what Shigeno has begun doing. At the Society for Neuroscience annual meeting in Chicago earlier this week, he presented preliminary findings from a series of experiments that investigated gene expression in the developing brain of Octopus bimaculoides, a small octopus that inhabits mud flats along the southern California coast. Shigeno found that several genes with well-known roles in patterning “lower” brain regions such as the brain stem and spinal cord in mice are expressed in a similar pattern in the developing octopus brain.

Genes involved in the development of “higher” brain regions in mice, on the other hand, showed more diverse patterns in the octopus brain. Genes expressed in the mouse hypothalamus, an area that regulates sleep and appetite, among other functions, also showed up in a lobe of the octopus brain thought to perform analogous tasks. But genes expressed in the mouse hippocampus and cerebral cortex, although also expressed in the octopus brain, had a distribution that bore little resemblance to their distribution in mice. To Shigeno, the findings suggest that lower levels of the octopus brain may be wired much like the analogous circuits in the vertebrate brain, but higher levels may show more divergence from the vertebrate wiring diagram.

Ragsdale agrees, but he cautions that such studies of gene expression tell only part of the story. He and Shigeno already have other experiments in the works to map out neural circuits in the octopus brain more directly and to study the neurochemistry of neurons in areas such as the hypothalamus analog to see if they secrete the same signaling peptides as do hypothalamic neurons in other animals. “We have a lot to learn about these animals,” Ragsdale says.

Photo credits: Cliff Ragsdale and Suichi Shigeno

by Mitch Leslie

Reptiles look old school, aslider(2).jpgnd they have old school B cells that retain an ancient ability our B cells have lost, says a new study published today. Our B cells cannot engulf invading bacteria, but a turtle's can. The results help narrow down when the immune system's antibody factories stopped dining out.

The mammalian immune system divvies up the pathogen-fighting duties. Macrophages and similar cells perform phagocytosis, eating and destroying bacteria and other invaders. Instead of tangling with pathogens, B cells counterattack by pumping out antibodies that home in on intruders. According to immunologists, mammalian B cells aren't capable of phagocytosis.

Researchers infer that phagocytosis came first—single-celled organisms such as amoebas use the maneuver to capture food—and that B cells evolved from phagocytic cells (see the Origins essay on the evolution of the immune system). A 2006 study bolstered that hypothesis, showing that cells from fish and frogs have both abilities.

The question is when in vertebrate evolution some B cells lost their appetite for pathogens. Nobody had run a taste test on reptile cells, so a team from Illinois State University in Normal offered fluorescent beads to B cells from red-eared sliders, a kind of turtle (above). Some of the cells snarfed up the beads, indicating that they were capable of phagocytosis, report Laura Zimmerman and colleagues in Biology Letters.

These dining habits suggest that the B cells of mammals didn’t give up phagocytosis until after the group parted from reptiles. The researchers propose that B cells' double duty reinforces immunity in reptiles, whose antibodies aren't as potent or produced as quickly as those of mammals. To sharpen their picture of B cell evolution, researchers now need to determine whether bird B cells also abstain from phagocytosis.

Photo credit: Trisha M. Sears

by Carl Zimmer

As I was working on my essay on the evolution of eukaryotes, I spoke a lot to Nick Lane. Lane is trained as a biochemist, but he's also a prolific author (most recently of the book Life Ascending). As a result, Lane is that particularly rare breed: a scientist who can not only offer a bird's-eye view of an entire field but also tell you about his own very interesting ideas. Now Lane has just won a prize to spend the next 3 years further exploring some of those ideas.

Lane is the first winner of the £150,000 Provost's Venture Research Prize, awarded by the University College London, where Lane has been an honorary reader since 2006. According to UCL, "the Provost's Venture Research Prize will go to UCL researchers whose ideas challenge the norm and have the potential to substantially change the way we think about an important subject."

Lane won the award for his proposal to tackle a few simple but profound questions:

Why have complex cells evolved only once in 4 billion years? Why do they share many unexpected traits like sex and senescence? If these traits offer a selective advantage, why do bacteria not take advantage? On current thinking, the answers to these questions should arise from genetics, but a narrowly genetic perspective suggests that complex life should evolve repeatedly.

Lane plans to flesh out a hypothesis I described in my essay: He suspects that a rare merging of two species made this complexity possible. Only after our single-celled ancestors engulfed bacteria (that are now mitochondria) did they get enough energy to build and run a complex cell.

"It will start out mostly theoretical," Lane wrote me in an e-mail. "I want to piece together a broad framework from the full breadth of the literature." Lane will then launch a series of experiments to test his idea—"through collaborations with labs who know what they're doing."

October 7, 2009

Yes, Ardi Evolved From Apes

by Ann Gibbons

ardicover “Ardi,” the oldest known skeleton of a hominin, or member of the human family, has grabbed headlines around the world since her unveiling in Science Thursday. Not surprisingly, the press coverage of the 4.4-million-year-old Ardipithecus ramidus has sometimes been sensational—and, in some cases, completely wrong.  Some newspapers and broadcasters have misinterpreted the authors’ finding that Ardi did not look like a chimpanzee or gorilla. Based on this anatomy, the authors proposed that Ardi shows that humans did not evolve from a “chimpanzee-like ape.” By that, they meant that Ardi evolved from an ancient ape that didn’t look like a chimpanzee or gorilla does today and that humans have retained some of those primitive traits.

But the word “chimpanzee-like” sometimes got lost in translation. Even the first version of a press release from Kent State University, where co-author C. Owen Lovejoy is on the faculty, said “Man Did Not Evolve From Apes.” And some media were clearly confused. The Torstar News Service in Canada wrote: “Man didn’t descend from apes. What is closer to the truth is that our knuckle-dragging cousins descended from us.”

A radio announcer in Baltimore, Maryland, asked me in an interview Monday if it was true that we were not apes—or even primates—and that we had our own, separate lineage that was more ancient. The same question came up in a Facebook chat with me and my editor at Science, Elizabeth Culotta, and has popped up in other media. 

Most disconcerting to the authors was the reporting on Ardi by the Arabic news network Al Jazeera, based in Doha, Qatar. A translation of the article written in Arabic starts with a headline that reads “Ardi Refutes Darwin’s Theory,” and the first sentence reads “American scientists have presented evidence that Darwin’s theory of evolution was wrong.” The article states that Ardi’s discovery “refutes the long-standing assumption that humans evolved from monkeys.”

Dr. Zaghloul El-Naggar, a professor of geology in several Arab universities (the article does not specify which ones), exclaims in the story that Westerners were beginning to “come to their senses after they used to deal with the origins of man from a materialistic perspective and by denying religions.” He goes on to claim that the age of Earth does not exceed 400,000 years, and that Ardi’s age of 4.4 million years is an exaggeration. 

For the record, all of this is plain wrong. Ardi is a primate descended from more ancient apes, as are all humans and human ancestors. Apes in turn are descended from monkeys. Chimpanzees are our closest living relatives— we share 96% of our DNA with them, and our lineages shared an ancestor sometime between 6 million and 8 million years ago, possibly earlier. The authors’ point is that the last common ancestor we shared with chimpanzees didn’t look like a chimp—which means that chimpanzees also have been evolving since the two lineages diverged. Finally, Ardi confirms rather than refutes Darwin’s prediction in 1871 that our progenitors lived on the African continent, as well as providing another link in the evolutionary chain from primitive apes to humans.


 by Elizabeth Pennisi

forestillustration

Why in tropical forests do tall broad-leaf trees tower over a layer of understory species? What dictates that shrubs and herbaceous plants pepper the ground below, creating an environment recognizable the world over as tropical forest.

Biologists have long wanted to know why forests and other ecological communities look the way they do. In the beginning of the 19th century, Prussian naturalist Alexander von Humboldt started to find out. He assembled the first comprehensive treatise on how vegetation varies with altitude, climate, soil, and other factors. The work was a groundbreaking exploration of the physical underpinnings of ecological structure: what determines the species that make up a community and their relative abundance.

More than a half-century later, Charles Darwin quietly conducted experiments in his garden at Down House that were even more seminal. Examining a patch of unkempt lawn as it went to seed, Darwin observed that the species changed through time and that competition led to the demise of less-vigorous ones.

Ever since, ecologists have wrestled with understanding what dictates the proportions of each plant in communities varying from meadows to montane forests. How these forces set up communities has "arguably been one of the most primary questions driving ecological science since its origins," says Brian Enquist of the University of Arizona, Tucson. Competition, predation, disturbance, and other factors have a heavy hand, and new research is showing the influential role of evolution, as well.

In this month's Origins essay, Erik Stokstad explores the thinking that has gone into understanding and interpreting community compositions. A combination of physical and biological forces organizes species into predictable communities such that a rainforest is recognizable no matter what part of the planet it grows in. He describes the role of these physical factors and the influence of different biological interactions. Some ecologists think competition is key; others contend the abundance and diversity of species in a community is determined mainly by random dispersal, speciation, and extinction. This latter idea, dubbed the "Unified Neutral Theory of Biodiversity," makes a radical assumption: It considers all organisms of the same trophic level (plants, say, or herbivores) as demographically identical; that is, each organism in a particular level has about the same chance of reproducing, dying, migrating, or giving rise to a new species. The neutral theory has had mixed support. Some researchers think that biological interactions and “neutral” factors work in concert. As challenging as sorting out the rules that govern ecological structure has been, the effort is worth it, say researchers, because of the potential conservation benefit.


Image: Katharine Sutliff