What Makes Us Different?

Sunday, Oct. 01, 2006 By MICHAEL D. LEMONICK AND ANDREA DORFMAN
ILLUSTRATION FOR TIME BY TIM O'BRIEN
Article Tools
Print
Reprints
Sphere
RSS

You don't have to be a biologist or an anthropologist to see how closely the great apes—gorillas, chimpanzees, bonobos and orangutans—resemble us. Even a child can see that their bodies are pretty much the same as ours, apart from some exaggerated proportions and extra body hair. Apes have dexterous hands much like ours but unlike those of any other creature. And, most striking of all, their faces are uncannily expressive, showing a range of emotions that are eerily familiar. That's why we delight in seeing chimps wearing tuxedos, playing the drums or riding bicycles. It's why a potbellied gorilla scratching itself in the zoo reminds us of Uncle Ralph or Cousin Vinnie—and why, in a more unsettled reaction, Queen Victoria, on seeing an orangutan named Jenny at the London Zoo in 1842, declared the beast "frightful and painfully and disagreeably human."

Related Articles

It isn't just a superficial resemblance. Chimps, especially, not only look like us, they also share with us some human-like behaviors. They make and use tools and teach those skills to their offspring. They prey on other animals and occasionally murder each other. They have complex social hierarchies and some aspects of what anthropologists consider culture. They can't form words, but they can learn to communicate via sign language and symbols and to perform complex cognitive tasks. Scientists figured out decades ago that chimps are our nearest evolutionary cousins, roughly 98% to 99% identical to humans at the genetic level. When it comes to DNA, a human is closer to a chimp than a mouse is to a rat.

Yet tiny differences, sprinkled throughout the genome, have made all the difference. Agriculture, language, art, music, technology and philosophy—all the achievements that make us profoundly different from chimpanzees and make a chimp in a business suit seem so deeply ridiculous—are somehow encoded within minute fractions of our genetic code. Nobody yet knows precisely where they are or how they work, but somewhere in the nuclei of our cells are handfuls of amino acids, arranged in a specific order, that endow us with the brainpower to outthink and outdo our closest relatives on the tree of life. They give us the ability to speak and write and read, to compose symphonies, paint masterpieces and delve into the molecular biology that makes us what we are.

Until recently, there was no way to unravel these crucial differences. Exactly what gives us advantages like complex brains and the ability to walk upright—and certain disadvantages, including susceptibility to a particular type of malaria, aids and Alzheimer's, that don't seem to afflict chimps—remained a mystery.

But that's rapidly changing. Just a year ago, geneticists announced that they had sequenced a rough draft of the chimpanzee genome, allowing the first side-by-side comparisons of human and chimpanzee DNA. Already, that research has led to important discoveries about the development of the human brain over the past few million years and possibly about our ancestors' mating behavior as well.

And sometime in the next few weeks, a team led by molecular geneticist Svante Paabo of the Max Planck Institute for Evolutionary Anthropology, in Leipzig, Germany, will announce an even more stunning achievement: the sequencing of a significant fraction of the genome of Neanderthals—the human-like species we picture when we hear the word caveman—who are far closer to us genetically than chimps are. And though Neanderthals became extinct tens of thousands of years ago, Paabo is convinced he's on the way to reconstructing the entire genome of that long-lost relative, using DNA extracted, against all odds, from a 38,000-year-old bone.

Laid side by side, these three sets of genetic blueprints—plus the genomes of gorillas and other primates, which are already well on the way to being completely sequenced—will not only begin to explain precisely what makes us human but could lead to a better understanding of human diseases and how to treat them.

FIRST GLIMMERINGS

Scientists didn't need to wait for the chimp genome to begin speculating about the essential differences between humans and apes, of course. They didn't even need to know about DNA. Much of the vitriol directed at Charles Darwin a century and a half ago came not from his ideas about evolution in general but from his insulting but logical implication that humans and the African apes are descended from a common ancestor.

As paleontologists have accumulated more and more fossils, they have compiled data on a long list of anatomical features, including body shape, bipedalism, brain size, the shape of the skull and face, the size of canine teeth, and opposable thumbs. Using comparative analyses of these attributes, along with dating that shows when various features appeared or vanished, they have constructed increasingly elaborate family trees that show the relationships between apes, ancient hominids and us. Along the way they learned, among other things, that Darwin, even with next to no actual data, was close to being right in his intuition that apes and humans are descended from a single common ancestor—and, surprisingly, that the ability to walk upright emerged millions of years before the evolution of our big brains.

But it wasn't until the 1960s that details of our physical relationship to the apes started to be understood at the level of basic biochemistry. Wayne State University scientist Morris Goodman showed, for example, that injecting a chicken with a particular blood protein from a human, a gorilla or a chimp provoked a specific immune response, whereas proteins from orangutans and gibbons produced no response at all. And by 1975, the then new science of molecular genetics had led to a landmark paper by two University of California, Berkeley, scientists, Mary-Claire King and Allan Wilson, estimating that chimps and humans share between 98% and 99% of their genetic material.