I’ve got a story on the cover of the latest issue of Time. It’s about the evolutionary origins of friendship. For a number of scientists, friendship–in a deep sense of the word–is not limited to our own species. The fact that friendship may be a widespread biological phenomenon could help us better understand why it has such a positive effect on our own health.

If you’re interested in the scientific literature, the best way in–and the way I first started to get familiar with it–is this review in the latest issue of Annual Review of Psychology by Dorothy Cheney and Robert Seyfarth, two of the world’s leading primatologists.

One thing that I delve into in the story is the question of just how widespread animal friendship really is. We don’t know, in large part because scientists haven’t done that many long-term field studies on wild animals. When scientists do watch dolphins or baboons for decades, they can see some bonds between unrelated individuals that last for long stretches. (Yet another value that comes from slow-cooked science.) On the other hand, what may look like friendship may just be anthropomorphic projection. In the article, I explain that a lot of cross-species “friendships” may be nothing like the kind seen in, say, chimpanzees. (As for the adorable dogs are on this week’s cover of Time, I note that the evidence about man’s best “friend” is quite thin.)

My story is behind a paywall, so you’ll need to subscribe or pick up a copy at a news stand. For a sense of the piece, here are the first few paragraphs–

Since 1995, John Mitani, a primatologist at the University of Michigan, has been going to Uganda to study 160 chimpanzees that live in the forests of Kibale National Park. Seventeen years is a long time to spend watching wild animals, and after a while it’s rare to see truly new behavior. That’s why Mitani loves to tell the tale of a pair of older males in the Kibale group that the researchers named Hare and Ellington.

Hare and Ellington weren’t related, yet when they went on hunting trips with other males, they’d share prey with each other rather than compete for it. If Ellington reached out a hand, Hare would give him a piece of meat. If one of them got into a fight, the other would back him up. Hare and Ellington would spend entire days traveling through the forest together. Sometimes they’d be side by side. Other times, they’d be 100 yards apart, staying in touch through the foliage with loud, hooting calls. “They’d always be yakking at each other,” says Mitani.

Their friendship—for that’s what Mitani calls it—lasted until Ellington’s death in 2002. What happened next was striking and sad. For all the years that Mitani had followed him, Hare had been a sociable, high-ranking ape. But when Ellington died, Hare went through a sudden change. “He dropped out,” says Mitani. “He just didn’t want to be with anybody for several weeks. He seemed to go into mourning.”

In 2011, the Loom reached its eighth birthday. Thanks to everyone who’s paid a visit or become a loyal reader in that time. With the year coming to a close, I spent a little time this week perusing the Loom’s archive, reflecting on the things that obsessed me during 2011.

More than many years, this one reminded me just how huge science is. Even if you limited yourself to the most important stories of this past year, there was just too much to keep up with. (Here’s Discover’s top 100 picks.) As a science writer, my focus is biology, but that didn’t ease my year-long case of head-spinning. The anchors that kept me from spinning away completely were the very small and the very complicated.

At the small end of the spectrum were, among other things, the bacteria that call us home. Like every year, 2011 saw outbreaks, such as the E. coli that sickened thousands in Germany. But now that we can read the genomes of these killers,  as I noted in Newsweek, we can see how chillingly fast new pathogens can evolve.

But the good germs also gained more recognition in 2011. The science of the microbiome is blooming at an astonishing pace, as you can see in the map I created for the September issue of Wired. As I got more familiar with the microbiome, it became clear to me that scientists won’t be able to handle its complexity without thinking like ecologists. I made that point in a talk this spring called “The Human Lake,” which I turned into a blog post in April. (I was delighted when it was selected as one of the best pieces of 2011 by The Browser and Longreads, and was picked to be including in the 2012 edition of Open Lab.)

The microbiome, I predict, is going to become very intimate in years to come. It’s a strangely thrilling experience to discover 53 species of bacteria living in one’s belly button, as I found out this year. In the future, doctors may check our bug types just as they check our blood types today. But all this new knowledge about the microbiome will bring us unexpected  ethical quandaries, some of which I discussed in December in the New York Times.

Bacteria may be small, but they’re positively plus-sized compared to viruses, the subject of my book A Planet of Viruses, which came out in May. (You can read excerpts in Audubon and i09.) Working on the book opened my eyes to just how abundant, diverse, and powerful viruses are–a point I tried to get across in the talks I gave in the spring. The two that I was happiest with were an interview on Science Friday on NPR, and a talk I gave at the Long Now Foundation in San Francisco. As always happens when I write a book about a fast-moving field, the science of virology offered up lots of surprises after the book came out–such as the biggest virus ever, a possible ancestor of hepatitis C in dogs, and signs of a battle between viruses and bacteria in our mouths. When the movie Contagion came out in September, I took a look in Slate at how realistic its story of a new world-wide pandemic was. I found it real enough to be very scary. And in an eerie bit of timing, this fall scientists developed a strain of bird flu that some researchers worry could make the movie a reality.

At the other end of the spectrum from bacteria and viruses is the human brain, those 100 billion neurons that make the universe aware of itself. There seems to be no end of revelatory research coming out of neuroscience and psychology. At the World Science Festival, I talked with three scientists doing extraordinary work on the mystery of sleep (you can watch the video here). In my own stories, I explored genes for language, teen brains, music in the brain, the neuroscience of smiles, how our brains make us capable of both war and peace, and the minds of Neanderthals. A lot of the pieces I wrote first appeared in the New York Times or magazines, but some of them have gotten a new lease on life. I published a new ebook in December, More Brain Cuttings, and my feature on the possibility of uploading our brains to achieve immortality was selected for The Best of American Science Writing 2011.

In 2011, it wasn’t just new science that was in the news. The nature of science was, too. Over the course of 2011, some high-profile papers came under fierce criticism, including arsenic-based life and a link between viruses and chronic fatigue syndrome. These studies prompted a debate about how science gets done in the first place, and how some of it then gets “de-discovered.” I pondered the nature of de-discovery in the New York Times in July, and the emergence of a more transparent discussion of science in Slate.

A lot of that discussion happened on Twitter. Twitter was just one of many new media that became more widespread this year. And just as scientists were getting comfortable with these channels of communication, science writers were too. I spent a fair amount of time in 2011 experimenting with different formats. On Twitter, I went after some egregiously bad science with a hashtag: #Greenfieldism. When I wasn’t on Twitter, I was often on Facebook, Tumblr, and Google+. Each medium has different strengths, I’ve found, which only emerge after playing around with it for a while. Google+ has spurred some fascinating discussions; Twitter is a fast way to spread links. I spent some time working with the folks at Radiolab this year, including the newly minted Macarthur genius Jad Abumrad. It was fascinating to see them turn spoken words into symphonies, such as this episode entitled “Patient Zero.” Another form of storytelling can be found at Story Collider, where people tell tales live in front of an audience. An invitation to be a part of a Story Collider evening led me to talk about how a trip to a war zone made me realize just how deeply science speaks to me. And at the end of the year I published Science Ink, a book born out of a blog-based obsession with science tattoos.

It was a strange year indeed when a traditional book felt like a fresh new format. And it makes me eager for the surprises waiting for us in 2012.

[Image: Wikipedia]

We take in streams of information of radically different forms: photons through the eyes, textures through the skin, air vibrations through the ears, molecules through the nose. Marvelously, we manage to integrate all that information into a unified, coherent feel of the world. It turns out that as we draw in these different streams, we use information from one sense to shape what we take in from others. It’s an efficient way to make the most of our imperfect perceptions. But it also leaves us vulnerable to some remarkable illusions, like the one illustrated in this video.

In my latest column for Discover, I explore our powers of multi-sensory integration. Check it out.

Last year I decided to play in the ebook sandbox. I brought together some of my favorite pieces about the brain in an anthology I entitled Brain Cuttings: Fifteen Journeys Through the Mind. I teamed up with the publishers George Scott and Charles Nix, and we produced an ebook.

Along the way, we learned a lot. I recounted some of the lessons in this piece for the Atlantic, and others in this conversation with the writer Steve Silberman. Suffice to say, publishing ebooks is by no means a frictionless utopia for writers. Nevertheless it remains strangely addictive. Perhaps we writers get the same jolt of dopamine that readers get when they tap a glass screen and are rewarded with a new book.

It just so happens I now have some new material to keep fueling my addition. I’ve continued to write about the brain, and recently I selected another crop of favorites. This new ebook has made it down the digital assembly line, and is now available for $7.99: More Brain Cuttings: Further Exporations of the Mind (Amazon, Barnes & Noble).

You’ll find a range of subjects here. How a 100 billion cells use 100 trillion connections to create a working brain. How the ringing in our ears may tell us important things about the nature of consciousness. How dancing cockatoos may reveal how we’re pre-adapted to love music.

I hope you enjoy the book. The brain unfolds like a flower; the more I have explored neuroscience, the more it has rewarded me with new stories. I expect there will be many more to come.

The New York Times has launched a series called Profiles in Science. When I was invited to join the undertaking, I proposed writing about the Harvard psychologist Steven Pinker. I had run into Pinker at the World Science Festival in June, and he had told me about his next book, The Better Angels of Our Nature, which was due out in the fall. In the 800+ page tome, Pinker argues that rates of human violence have been crashing for millennia, and he offers psychological explanations for the fall.

I’ve followed Pinker’s work since I first came across his 1994 book, The Language Instinct. In the wake of the book’s success, he quickly became a leading exponent of evolutionary psychology, coming out swinging against its critics such as Stephen Jay Gould. When Pinker described his book to me, I was intrigued. I wondered how someone who argued that human nature was shaped long ago by natural selection would end up arguing that human nature–or at least human experience–is now changing rapidly for the better. But there were other things I was wondering–how, for example, does a writer of massive books about human nature live inside the same body as an expert on irregular verbs?

So I headed up to Cambridge to ask a bunch of questions, out of which a profile emerged. You can read it in tomorrow’s Times, or on their web site.

At the site, you can also watch a video interview with Pinker from NYT senior producer Thomas Lin.

We know that our species is unique, but it can be surprisingly hard to pinpoint what exactly makes us so. The fact that we have DNA is not much of a mark of distinction. Several million other species have it too. Hair sets us apart from plants and mushrooms and reptiles, but several thousand other mammals are hairy, too. Walking upright is certainly unusual, but it doesn’t sever us from the animal kingdom. Birds can walk on two legs, after all, and their dinosaur ancestors were walking bipedally 200 million years ago. Our own bipedalism–like much of the rest of our biology–has deep roots. Chimpanzees, whose ancestors diverged from our own some seven million years ago, can walk upright, at least for short distances.

If looking for human uniqueness on the outside is difficult, is it any easier to look on the inside–in particular, at our mental lives? There’s no doubt that our minds allow us to do things that even our great ape relatives cannot. For one thing, we can represent the world symbolically in our heads, and we can use words to communicate that symbolic thought to one another. Yet we can sometimes find surprising links between our own mental lives and those of other animals. We’re very good at making and using tools, but that doesn’t mean other animals can’t do so as well. Thinking about the future may seem like a quintessentially human activity, but there’s some evidence that some bird species can travel forward in time, too.

Yet even as scientists find more links between our own faculties and those of other animals, some continue to stand out. And their rugged distinctiveness makes them all the more interesting. One of the most distinctive of all is, to me at least, the most surprising: teaching. (more…)

Our brains are protected by an invisible fortress wall, keeping it safe from many dangers. Unfortunately, it also keeps out a lot of the drugs that could help cure diseases of the brain. In this month’s column for Discover, I look at some of the newest strategies for scaling the wall. Check it out.

Image: Ken Lund, Flickr, via Creative Commons

When the Society for Neuroscience gets together for their annual meeting each year, a city of scientists suddenly forms for a week. This year’s meeting has drawn 31,000 people to the Washington DC Convention Center. The subjects of their presentations range from brain scans of memories to the molecular details of disorders such as Parkinson’s and autism. This morning, a scientist named Svante Paabo delivered a talk. Its subject might make you think that he had stumbled into the wrong conference altogether. He delivered a lecture about Neanderthals.

Yet Paabo did not speak to an empty room. He stood before thousands of researchers in the main hall. His face was projected onto a dozen giant screens, as if he were opening for the Rolling Stones. When Paabo was done, the audience released a surging crest of applause. One neuroscientist I know, who was sitting somewhere in that huge room, sent me a one-word email as Paabo finished: “Amazing.”

You may well know about Paabo’s work. In August, Elizabeth Kolbert published a long profile in the New Yorker. But he’s been in the news for over fifteen years. Like many other journalists, I’ve followed his work since the mid-1990s, having written about pieces of Paabo’s work in newspapers, magazines, and books. But it was bracing to hear him bring together the scope of his research in a single hour–including new experiments that Paabo’s colleagues are presenting at the meeting. Simply put, Paabo has changed the way scientists study human evolution. Along with fossils, they can now study genomes that belonged to people who died 40,000 years ago. They can do experiments to see how some of those individual genes helped to make us human. During his talk, Paabo used this new research to sketch out a sweeping vision of how our ancestors evolved uniquely human brains as they swept out across the world.

Before the 1990s, scientists could only study the shape of fossils to learn about how we evolved. A million years ago, the fossil record contained evidence of human-like creatures in Europe, Asia, and Africa. Roughly speaking, the leading hypotheses for how those creatures became Homo sapiens came in two flavors. Some scientists argued that all the Old World hominins were a single species, with genes flowing from one population to another, and together they evolved into our species. Others argued that most hominin populations became extinct. A single population in Africa evolved into our species, and then later spread out across the Old World, replacing other species like Neanderthals in Europe.

It was also possible that the truth was somewhere in between these two extremes. After our species evolved in Africa, they might have come into contact with other species and interbred, allowing some DNA to flow into Homo sapiens. That flow might have been a trickle or a flood.

As scientists began to build a database of human DNA in the 1990s, it became possible to test these ideas with genes. In his talk, Paabo described how he and his colleagues managed to extract some fragments of DNA from a Neanderthal fossil–by coincidence, the very first Neanderthal discovered in 1857. The DNA was of a special sort. Along with the bulk of our genes, which are located in the nucleus of our cells, we also carry bits of DNA in jellybean-shaped structures called mitochondria. Since there are hundreds of mitochondria in each cell, it’s easier to grab fragments of mitochondrial DNA and assemble them into long sequences. Paabo and his colleagues used the mutations in the Neanderthal DNA, along with those in human and chimpanzee DNA, to draw a family tree. This tree splits into three branches. The ancestors of humans and Neanderthals branch off from the ancestors of chimpanzees 5-7 million years ago, and then humans and Neanderthals branch off in the last few hundred thousand years. If humans carried mitochondrial DNA from Neanderthals, you’d expect Paabo’s fossil genes to be more similar to some humans than others. But that’s not what he and his colleagues found.

Paabo and his colleagues then pushed forward and began to use new gene-sequencing technology to assemble a draft of the entire Neanderthal genome. They’ve gotten about 55% of the genome mapped, which is enough to address some of the big questions Paabo has in mind. One is the question of interbreeding. Paabo and his colleagues compared the Neanderthal genome to genomes of living people from Africa, Europe, Asia, and New Guinea. They discovered that people out of Africa share some mutations in common with Neanderthals that are not found in Africans. They concluded that humans and Neanderthals must have interbred after our species expanded from Africa, and that about 2.5% of the genomes of living non-Africans comes from Neanderthals.

This pattern could have arisen in other ways, Paabo granted. The ancestors of Neanderthals are believed to have emerged from Africa hundreds of thousands of years ago and spread into Europe. Perhaps the humans who expanded out of Africa came from the birthplace of Neanderthals, and carried Neanderthal-like genes with them.

But Paabo doubts this is the case. One way to test these alternatives is to look at the arrangement of our DNA. Imagine that a human mother and Neanderthal father have a hybrid daughter. She has two copies of each chromosome, one from each species. As her own eggs develop, however, the chromosome pairs swap some segments. She then has children with a human man, who contributes his own human DNA. In her children, the Neanderthal DNA no longer runs the entire length of chromosomes. It forms shorter chunks. Her children then have children; her grandchildren have even shorter chunks.

Paabo described how David Reich of Harvard and other scientists measured the size of the chunks of Neanderthal DNA in people’s genomes. They found that in some of the Europeans they studied, the Neanderthal chunks were quite long. Based on their size, the scientists estimated that the interbreeding happened between 37,000 and 86,000 years ago. (This research is still unpublished, but Reich discussed it at a meeting this summer.)

The success with the Neanderthal genome led Paabo to look for other hominin fossils that he could grind up for DNA. DNA probably can’t last more than a few hundred thousand years before degrading beyond recognition, but even in that window of time, there are plenty of interesting fossils to investigate. Paabo hit the jackpot with a tiny chip from the tip of a 40,000-year-old pinky bone that was found in a Siberian cave called Denisova. The DNA was not human, nor Neanderthal. Instead, it belonged to a distant cousin of Neanderthals. And when Paabo and his colleagues compared the Denisovan DNA to human genomes, they found some Denisovan genes in the DNA of their New Guinea subject. Mark Stoneking, Paabo’s colleague at Max Planck, and other scientists have expanded the comparison and found Denisovan DNA in people in Australia and southeast Asia.

Paabo then offered a scenario for human evolution: about 800,000 years ago, the ancestors of Neanderthals and Denisovans diverged from our own ancestors. They expanded out of Africa, and the Neanderthals swept to the west into Europe and the Denisovans headed into East Asia. Paabo put the date of their split about 600,000 years ago. The exact ranges of Neanderthal and Denisovans remain fuzzy, but they definitely lived in Denisova at about the same time 50,000 years ago, given that both hominins left bones in the same cave.

Later, our own species evolved in Africa and spread out across that continent. Humans expanded out of Africa around 100,000 years ago, Paabo proposed. (I’m not sure why he gave that age, instead of a more recent one.) Somewhere in the Middle East, humans and Neanderthals interbred. As humans continued to expand into Europe and Asia, they took Neanderthal DNA with them. When humans got to southeast Asia, they mated with Denisovans, and this second addition of exotic DNA spread through the human population as it expanded. Neanderthals and Denisovans then became extinct, but their DNA lives on in our bodies. And Paabo wouldn’t be surprised if more extinct hominins turn out to have donated DNA of their own to us.

Paabo sees these results as supporting the replacement model I described earlier–or, rather, a “leaky replacement” model. If humans and other hominins had been having lots of sex and lots of kids, we’d have lots more archaic DNA in our genomes.

Now that scientists know more about the history of our genome, they can start tracking individual genes. When I first wrote about this interbreeding work last year for the New York Times, I asked Paabo if there were any genes that humans picked up from interbreeding that made any big biological difference. He didn’t see any evidence for them at the time. But at the meeting, he pointed to a new study of immune genes. One immune gene appears to have spread to high frequency in some populations of Europeans and Asians, perhaps because it provided some kind of disease resistance that benefited them.

The history of other genes is just as interesting. Some of our genes have mutations also found in Neanderthals and Denisovans, but not in chimpanzees. They must have evolved into their current form between 5 million and 800,000 years ago. Other genes have mutations that are found only in the human genome, but not in those of Neanderthals and Denisovans. Paabo doesn’t have a complete list yet, since he’s only mapped half the Neanderthal genome, but the research so far suggests that the list of new features in the human genome will be short. There are only 78 unique human mutations that changed the structure of a protein. Paabo can’t yet say what these mutations did to our ancestors. Some of the mutations alter the address labels of proteins, for example, which let cells know where to deliver a protein once they’re created. Paabo and his colleagues have found that the Neanderthal and human versions of address labels don’t change the delivery.

Other experiments Paabo and his colleagues have been running have offered more promising results. At the talk, Paabo described some of his latest work on a gene called FoxP2. Ten years ago, psychologists discovered that mutations to this gene can make it difficult for people to speak and understand language. (Here’s a ten-year retrospective on FoxP2 I wrote last month in Discover.) Paabo and his colleagues have found that FoxP2 underwent a dramatic evolutionary change in our lineage. Most mammals have a practically identical version of the protein, but ours has two different amino acids (the building blocks of proteins).

The fact that humans are the only living animals capable of full-blown language, and the fact that this powerful language-linked gene evolved in the human lineage naturally fuels the imagination. Adding fuel to the fire, Paabo pointed out that both Neanderthals and Denisovans had the human version of FoxP2. If Neanderthals could talk, it would be intriguing that they apparently couldn’t paint or make sculptures or do other kinds of abstract expressions that humans did. And if Neanderthal’s couldn’t talk, it would be intriguing that they already had a human version of FoxP2. As scientific mysteries go, it’s a win-win.

From a purely scientific point of view, the best way to investigate the evolution of FoxP2 would be to genetically engineer a human with a chimpanzee version of the gene and a chimpanzee with a human version. But since that’s not going to happen anywhere beyond the Island of Doctor Moreau, Paabo is doing the second-best experiment. He and his colleagues are putting the human version of FoxP2 into mice.

The humanized mice don’t talk, alas. But they do change in many intriguing ways. The frequency of their ultrasonic squeaks changes. They become more cautious about exploring new places. Many of the most interesting changes happen in the brain. As I wrote in my Discover column, Paabo and his colleagues have found changes in a region deep in the brain called the striatum. The striatum is part of a circuit that lets us learn how to do new things, and then to turn what we learn into automatic habits. A human version of FoxP2 makes neurons in the mouse striatum sprout more branches, and those branches become longer.

Paabo’s new experiments are uncovering more details about how human FoxP2 changes the mice. Of the two mutations that changed during human evolution, only one makes a difference to how the striatum behaves. And while that difference may not allow mice to recite Chaucer, they do change the way they learn. Scientists at MIT, working with Paabo, have put his mice into mazes to see how quickly they learn how to find food. Mice with human FoxP2 develop new habits faster than ones with the ordinary version of the gene.

So for now, Paabo’s hypothesis is that a single mutation to FoxP2 rewired learning circuits in the brain of hominins over 800,000 years ago. Our ancestors were able to go from practice to expertise faster than earlier hominins. At some point after the evolution of human-like FoxP2, our ancestors were able to use this fast learning to develop the quick, precise motor control required in our lips and tongues in order to speak.

I think what made Paabo’s talk so powerful for the audience was that he was coming from a different world–a world of fossils and stone tools–but he could talk in the language of neuroscience. As big as the Society for Neuroscience meetings can be, Paabo showed that it was part of a much bigger scientific undertaking: figuring out how we came to be the way we are.

[Image: Frank Vinken]

Ten years ago this month, a team of University of Oxford scientists published a description of a family who struggled with words. By comparing their DNA, the scientists zeroed in for the first time on a gene associated with language, dubbed FOXP2. In my newest column in Discover, I look back at what scientists have learned over the past decade about how FOXP2 works, and what it tells us–or leaves us wondering–about how language evolved. Check it out.

Metaphors are essential to writing about science. Even scientists themselves use metaphors all the time, drawing from their familiar experiences to describe the unfamiliar. Building proteins is known as translation, for example, because the sequences of DNA and proteins are akin to words written in different languages. The cell has to translate one language into another using–another metaphor–the genetic code.

Metaphors can be powerful, but they can also trip us up if we mistake them for an equivalence. DNA isn’t really a human language, for example. In my latest column for Discover, I take a look at another tricky metaphor: the eye as camera. Some scientists are actually making that metaphor real, by building video cameras that can let blind people see. As I point out, however, eyes are not cameras, and the differences are fascinating. They’re also crucial to the future success in treating blindness with technology. Check it out. 

If you were this man, you’d be smiling too.

The man is Lee Berger, a paleoanthropologist at the University of Witwatersrand in South Africa. He’s holding the skull of Australopithecus sediba, a 1.98 million year old relative of humans, otherwise known as a hominin. In April 2010 Berger and his colleagues first unveiled the fossil in the journal Science. As I wrote in Slate, Berger argued that A. sediba was the closest known cousin to our genus Homo. Hominins branched off from other apes about 7 million years ago, but aside from becoming bipedal, they were remarkably like other apes for about five million years. Among other things, they were short, had long arms, and had small brains. Berger and his colleagues saw in A. sediba what biologists often find in transitional forms–a mix of ancestral and newer traits. It has Homo-like hands, a projecting nose, and relatively long legs. It was intermediate in heigh between earlier hominins and the tall Homo. And it still had a small brain and long arms. (In August, Josh Fishman wrote a feature for National Geographic on A. sediba, complete with excellent reconstructions.)

It wasn’t just finding such a potentially significant fossil that would make you smile if you were Lee Berger. It’s how much stuff he and his colleagues have found. The skull that Berger holds would be enough to keep several scientists busy for years. But Berger and his team have much more. In fact, A. sediba is, in some ways, now even better represented than far more recent hominin relatives.

Today, Science has turned over much of this week’s issue to follow-up papers from Berger’s team, in which they share some of the goodies. Here, for example, is A. sediba’s hand. Before this specimen came to light, paleoanthropologists had much less to look at to study the origin of the human hand. The best specimen came from a 1.75 million year old hominin called Homo habilis. It got the name Homo in part because the fossils were found along with stone tools, which were considered a sign of a very human-like creature. Researchers also found bones from its hand–but only 13 fragments. In this picture of A. sediba‘s hand, just about every bone is real. This is what paleoanthropologists dream about at night.

Tracy Kivel, a paleoanthropologist at the Max Planck Institute, led a team of researchers who compared A. sediba’s hand to the hands of humans, chimps, gorillas, and extinct hominins. They found even more mingling of old and new traits than before. The hand has ridges for powerful muscles that run up the length of the hand. Chimpanzees have muscles like these, which give them stronger grips as they climb around in trees. Earlier hominins have them too. We don’t. Instead, we have long thumbs and fleshy pads on our finger tips, which are great if you’ve come to depend on your skill to make and use tools. A. sediba has them too.

Scientists have found likely hominin stone tools dating back 2.6 million years ago; last year a team of researchers kicked up some controversy by claiming to have found signs of stone tools 3.4 million years ago. It’s clear that by the time A. sediba came on the scene, hominins had been using stone tools for hundreds of thousands of years. It’s too bad that Berger and his colleagues haven’t found any tools alongside A. sediba’s bones, to see what they could do with these transitional hands. Then again, why should he get all the fun?

Things got particularly intriguing when Kivel and company compared A. sediba‘s hand to Homo habilis’s. Remember, Homo habilis is about 250,000 years younger than A. sediba. Yet A. sediba‘s hand is actually more like our own than that of Homo habilis. It’s got some wrist bones that are shaped to handle strong forces transmitted from the thumb–the sort of forces you might expect from whacking stones together to make a cleaver, for example. Evidence such as this suggests that Homo habilis branched off first from the ancestors of A. sediba and later hominins like ourselves, and then later A. sediba branched off from our own lineage. Along the way, the hand gradually became less adapted for tree-climbing, and acquired more traits we use to handle tools.

In other papers, scientists take a look at A. sediba‘s brain and hips. The two are more intimately associated than you might think at first. We have huge brains even at birth, which make child-bearing a tricky proposition in our species, because they have to be able to pass through the birth canal. We humans have wide hips compared to other apes, and some researchers have argued that they evolved in tandom with our expanding brains. (See this column I wrote recently for more compensations in our bodies for big brains.) But it turns out that A. sediba–which had a small brain–already had broader hips than earlier hominins. Whatever drove its hip expansion, a big head wasn’t it.

While the A. sediba brain was small, it demonstrates that in hominin brain evolution, size isn’t everything. The skull Berger holds here contains a beautifully preserved cavity inside. When he and his colleagues put the fossil in a scanner, they were able to reconstruct the shapes of a lot of the left hemisphere of the brain and the front chunk of its right. The shapes of some parts of the brain (in particular, a part of the brain called orbitofrontal cortex) are more like our own than like earlier hominins.

Reading this, I can’t help but dabble in a little paleo-phrenology. The orbitofrontal cortex is a crucial node in our emotional network, where neurons assign value to things and can tamp down or ramp up our automatic responses of fear and delight. Did a glimpse of human feelings mark this great transition, long before human-sized brains evolved?

I doubt scientists will ever answer that question, but not to worry: there are many more answers A. sediba will be able to provide.

[Images: Berger, courtesy of Lee Berger and University of Witwatersrand; hand and pelvis by Peter Schmid, courtesy of Lee Berger and University of Witwatersand; brain, photo by ESRF/KJ Carlson, courtesy of Lee Berger and the University of Witwatersrand]

In tomorrow’s New York Times, I take a look at a new study on the generosity of chimpanzees. Check it out. (And also check out Ed Yong’s take at Not Exactly Rocket Science.)

[Image courtesy of Frans de Waal]