THE NEXT TIME YOU VISIT A ZOO, WALK PAST THE ape cages. Imagine that the apes had lost most of their hair and that next to them was a cage holding some unfortunate people who had lost their clothes and couldn’t speak but were normal in every other way. Now try guessing how different the apes’ genes are from the humans’ genes. Would you guess that a chimpanzee shares 10 percent, 50 percent, or 99 percent of its genetic makeup with humans?
In recent decades, science has answered that question. Even though many other questions remain unanswered, we now know more about our origins than ever before. Every human society has felt a deep need to make sense of its origins, and has met that need with its own story of creation. The creation story of our time is the tale of three chimps.
For centuries it’s been clear roughly where we fit into the animal kingdom. We are mammals, part of the group of animals that have hair and nurse their young. Among mammals, we are primates, the group of mammals that includes monkeys and apes. We share primate features that most non-primates do not have, including flat fingernails and toenails (rather than claws), hands for gripping, and thumbs that can move in the opposite direction from our fingers.
Within the primates, we are more similar to apes (gorillas, chimpanzees, orangutans, and gibbons) than to monkeys. For one thing, monkeys have tails, but apes and humans do not. Gibbons stand out from the other apes because they are small and have very long arms. Gorillas, chimpanzees, orangutans, and humans are more closely related to one another than any of them is to gibbons.
Going further into our primate relationships proved difficult for scientists. It led to an intense debate centered on three questions:
* What is the detailed family tree of relationships among humans, the living ape species, and the extinct ape species that were our ancestors? If we knew the answer to this question, we would know which living ape is our closest relative.
* When did we and our closest living relative last share the same ancestor? This would tell us how long ago the human line branched off the family tree.
* How much of our genetic makeup do we share with our closest living relative? This would tell us what percentage of our genes is uniquely human.
Fossil evidence might answer the first two questions, except for one unfortunate fact. Almost no ape fossils have been found for the crucial period between five and fourteen million years ago in Africa. Instead, the answers to the questions came from an unexpected source: a project to sort out relationships among bird species.
A Clue from the Bird World
In the 1960s, molecular biologists began to realize that the chemicals that make up plants and animals might provide “clocks” to measure the genetic distances between species, and to tell how long ago those species separated from each other on the evolutionary tree. Take lions and tigers, for example.
Suppose we knew from fossils that lions and tigers separated five million years ago. Suppose that a certain molecule in lions was 1 percent different from the same molecule in tigers. That would mean that 1 percent of genetic difference equaled five million years of separate evolution. Then, if scientists wanted to compare two living species but had no fossils to show those species’ evolutionary history, they could look at that same molecule in both species. If the difference between the two molecules was 3 percent, they would know that the species separated from their shared ancestor about fifteen million years ago—that is, three times five million.
In the 1970s, two scientists named Charles Sibley and Jon Ahlquist used the idea of a molecular clock based on changes in DNA to study the evolutionary relationships of about 1,700 bird species, nearly a fifth of all living birds. A decade later, they used the same techniques to study primate evolution. For this project they examined the DNA of humans and all our closest relatives: common chimpanzees, bonobos (or pygmy chimpanzees), gorillas, orangutans, two species of gibbons, and seven species of monkeys. Their results gave us a new understanding of the primate family tree.
A CLOCK MADE OF DNA
THE MOLECULAR CLOCK WORKS THIS WAY:
Suppose some type of molecule existed in all species, but had a unique structure in each species. Suppose that structure changed slowly over millions of years because of genetic mutations, and suppose the rate of change was the same for all species.
Two species that descended from the same ancestor would start off with identical forms of the molecule, inherited from their ancestor. Over time, though, mutations would occur independently in each line of descent. These mutations would change the structure of that molecule in each of the two species. We could measure the present difference in the molecule’s structure between the two. Then, if we knew how many structural changes occurred, on average, every million years, the present difference between the two species would serve as a “clock,” telling us how much time had passed since the two species shared a common ancestor.
By around 1970, molecular biologists had found that the best “clock” molecule is deoxyribonucleic acid, or DNA. It occurs in all living things but is unique to each species. DNA is made up of two long chains of molecules.
Each chain is made up of four types of small molecules. The sequence, or order, of those small molecules carries all the genetic information that is passed from parents to offspring.
To measure changes in DNA structure, scientists use a method called DNA hybridization. They mix the DNA from two species, then measure the melting point of this mixed, or hybrid, DNA. The next step is to compare the melting point of the hybrid DNA with the melting point of pure DNA from a single species. A difference of about one degree centigrade means that the two species differ by about 1 percent of their DNA.
The final stage is to calibrate, or set, the DNA clock. This means linking DNA change to the passage of time. We might know that two species’ DNA differs by 1 percent, but until we know how DNA changes over time, we can’t know how long the two species have been evolving separately. To calibrate the DNA clock, scientists use species whose evolutionary history is known from fossils that can be accurately dated. In the case of birds, studies of both fossils and DNA from living bird species reveal that one gene in DNA (the gene called cytochrome b) appears to change by 1 percent every one million years. Using this information, scientists can measure the differences in cytochrome b in any two living bird species and tell how long ago those species separated from the ancestor they shared.
A key to understanding connections among living things lies in DNA, the genetic material within our cells. It is made up of two long strands of molecules linked by shorter pairs of molecules, like a ladder with many rungs that has been twisted into a spiral—a shape known as a double helix.
FIGURE I
A family tree of evolutionary relationships among primates, including humans. Black dots represent the last time two groups shared the same ancestor. The scale on the right measures time and the scale on the left measures differences in the DNA of living species. Start with the dot on the bottom right—that’s the point, more than 30 million years ago, when apes separated from the monkeys of Eurasia and Africa. The monkeys kept evolving right up to the present. As the apes evolved, gibbons split off into their own line about 20 million years ago. The black dot marking that split is at 5 percent on the DNA scale, because gibbons differ in 5 percent of their DNA from other apes and humans. The second black dot from the left shows humans and chimpanzees splitting about 7 million years ago, with a difference of less than 2 percent in the DNA of humans and chimpanzees today.
When scientists studied the molecular clock in primate DNA, they found that the biggest genetic difference is between monkeys on one hand and apes and humans on the other. This came as no surprise. Ever since apes became known to science, everybody has agreed that humans and apes are more closely related to each other than either of them is to monkeys. The molecular clock showed that monkeys differ from humans and apes in 7 percent of their DNA structure.
The clock also confirmed that gibbons are the most distinct apes. They differ from the other apes and humans in 5 percent of their DNA structure. Orangutans differ in 3.6 percent from gorillas, chimpanzees, and humans. These findings show that gibbons and orangutans separated from the rest of the ape family long ago. Today, gibbons and orangutans are found only in Southeast Asia. In contrast, gorillas and chimpanzees are found only in Africa, which was also the home of the earliest humans. Among apes, the most closely related living species are the two types of chimpanzees, common chimps and bonobos. Their DNA is 99.3 percent identical.
What about humans? We differ from gorillas in about 2.3 percent of our DNA, and from chimps of both species by about 1.6 percent. This means that we share 98.4 percent of our DNA with chimpanzees, our closest living relatives. Put another way, the chimpanzee’s closest relative is not the gorilla—it is the human.
Calibrated for primate species, the DNA clock shows that gorillas separated from the line leading to chimps and humans about ten million years ago. Ancestral humans separated from chimps about seven million years ago. in other words, humans have been evolving on their own for something like seven million years.
The genetic distance separating us from chimps is less than the distance between two species of gibbons (2.2 percent). in an example from the bird world, the red-eyed vireo and white-eyed vireo are species of songbirds. Both belong to the same genus, or cluster of closely related species. But they differ in 2.9 percent of their DNA—much more than the difference between us and chimps. in terms of genetic distance, humans, common chimps, and bonobos should be grouped in the same genus. Looked at this way, humans are a third species of chimpanzee.
HOW SHOULD WE TREAT APES?
NOW THAT WE KNOW HOW SMALL A GENETIC distance separates us from chimps, our ideas about the places of humans and apes may change over time. One area that might change is the way we treat apes. Ethical issues—that is, questions of what is right and wrong—are involved.
It’s considered acceptable to exhibit caged apes in zoos, but it’s not acceptable to do the same thing with humans. Yet if it were not for the interest in apes that many people gain by visiting zoos, the public might contribute much less money to protect apes in the wild. How will we balance our desire to collect chimpanzees and other apes in zoos with our knowledge that we are so closely related to chimps?
Medical experimentation on chimpanzees is a controversial subject. It is unethical, or wrong, to perform experiments on humans without their knowledge and consent. Why is it okay to perform such experiments on chimps? If we say it is because chimps are animals, then we are saying they are no different from insects and bacteria in terms of what we can do with them, because insects and bacteria are also animals. But if we consider intelligence, social organization, and the ability to feel pain, it becomes hard to draw an all-or-nothing line between all humans and all animals. Instead, different ethical rules should apply to research on different species. If there is any animal species now used in medical research for which we could argue that all experimentation should stop, that species is surely the chimpanzee.
Making matters worse, chimps used for research are often caged under cruel conditions. The first research chimp that I saw had been injected with a slow-acting deadly virus. It was being kept alone for several years, in a small indoor cage, with nothing to play with, until it died. And the capture of wild chimps for research usually means that several wild chimps are killed to capture one, which is often a youngster being carried by its mother.
Yet the very reason medical researchers use chimps is that they are so genetically similar to us. Experiments on apes are a far better way to improve medical treatments than experiments on any other animals. Researchers are now studying certain diseases in captive chimps. How would we explain to parents whose children are at risk of dying from those diseases that their children are less important than chimps? Ultimately, we the public, not just scientists, will have to make these terrible choices. Our view of man and apes will determine our decision.
The Differences between Chimps and Humans
How could just 1.6 percent of genetic difference change chimpanzees into humans? Exactly which genes changed? To answer those questions we need to understand what DNA, our genetic material, does.
Much of our DNA has no known function. Of the DNA that does have known functions, the main functions have to do with proteins, which are long chains of amino acids. Parts of our functional DNA govern the creation of proteins. It works this way: The sequences of small molecules in our DNA specify, or direct, the order of amino acids in our proteins. Certain proteins make up our hair and tissue, while other proteins are enzymes that make and break down the other molecules in our bodies.
The genetic features that are easiest to understand arise from single proteins and single genes, or chunks of DNA. For example, our blood’s oxygen-carrying protein, hemoglobin, is made of two amino acid chains, each specified by a single gene. But other genes influence more than one trait. For example, the fatal genetic disease Tay-Sachs causes many visible traits: drooling, abnormal skull growth, yellowish skin, and more. We know that all these effects come from changes in a single enzyme specified by the Tay-Sachs gene, but we don’t know how.
Scientists understand the functions of many individual genes that specify individual proteins, but we know much less about how genes shape complex traits, such as behavior. Human hallmarks—that is, characteristics that distinguish us as human—such as art, language, or aggression, are not likely to depend on a single gene. in addition, human behavior is influenced by family, culture, nutrition, and other aspects of each person’s environment. it’s very controversial what role genes play in the individual differences among humans. But for differences in behavior between all chimpanzees and all humans, genetic differences are likely to play a role.
The ability of humans but not chimps to speak, for example, must be related to differences in genes that specify the structure of the voice box (larynx) and the wiring of the brain. A young chimpanzee that was raised in a psychologist’s home along with the psychologist’s daughter of the same age didn’t learn to speak or walk upright, although the girl did. Humans grow up to speak, no doubt because of our genetic program. But whether an individual human grows up to speak english or Korean has nothing to do with genes. it depends on what languages the growing child hears spoken.
We don’t yet know which chunks of our DNA are responsible for the significant differences between humans and chimps that are covered in the next four chapters. All we can say for sure is that those differences must come from some part of 1.6 percent of our genes. We do know that just one or a few genes can have big impacts. The many big, visible differences between Tay- Sachs patients and those without the disease come from one change in one enzyme.
Cichlid fish, popular for aquariums, also show the impact of small genetic changes. Africa’s Lake victoria has about two hundred species of cichlids. All of them evolved from a single ancestor over a period of about two hundred thousand years. These species differ in their food habits as much as tigers and cows do. Some cichlids graze on algae, some catch insects, some nibble the scales off other fish, some crush snails, and some snatch fish embryos from mother fish. Yet all those species differ from one another by less than half of 1 percent of their DNA. it took fewer genetic mutations to turn a snail crusher into a baby snatcher than it took to produce us from an ape.
