Nothing gets the blood boiling like a manimal. For many people, the idea of breaching the human species barrier--to mingle our biology with that of an animal--seems like a supreme affront to the moral order. In his January state of the union address, President Bush called for a ban on "creating human-animal hybrids." These so-called chimeras, according to their opponents, devalue humanity by breaching our species barrier. "Human life is a gift from our creator, and that gift should never be discarded, devalued or put up for sale," Bush declared. Senator Sam Brownback of Kansas expanded on this sentiment in his Human Chimera Prohibition Bill of 2005. Chimeras, according to the bill, "blur the lines between human and animal." They must be banned because "respect for human dignity and the integrity of the human species may be threatened by chimeras." Some opponents cite the Bible as proof that chimeras are wrong--in particular, I Corinthians 15:39: "All flesh is not the same flesh: but there is one kind of flesh of men, another flesh of beasts, another of fishes, and another of birds." Others rely on their own sense of disgust as a reliable guide to the wrongness of chimeras. "When we start to blend the edges of things, we're uneasy," explains Grant Hurlburt, a psychiatrist and member of the President's Council on Bioethics. "That's why chimeric creatures are monsters in mythology in the first place." So let's imagine, for the sake of argument, that a nefarious plot to create human-ape hybrids was discovered in some distant country. Genetic studies revealed at some point in the past, the DNA of that nation's citizens had been mixed with the DNA from a separate species of ape. The species barrier had been breached, and the contamination had been carried down from generation to generation. Even today a significant amount of DNA in people of that nation could be traced to that hybridization. Horror and condemnation would arise at the discovery that chimeras had been created. But of course, this is all hypothetical, right? Perhaps not. Scientists know a fair amount about how species split apart--including the genetic changes that occur during the split. An old species splits into new ones when one of its populations becomes isolated. The individuals in a population begin to interbreed more with one another than with the rest of the species. When mutations emerge in that population, they don't spread to the rest of the species. Over time, the population becomes more and more genetically distinct from other members of the species. They may begin to look different, and they may begin to specialize in different ways of getting food. They may still be able to interbreed with other members of their species, at least in theory. But barriers may emerge. A population of plants may begin to flower at a different time of year, for example. Fish may prefer to mate with fish that look more like themselves. If the population has some contact with the rest of its species, a few hybrids may be produced in each generation. In some cases, they more form a healthy hybrid zone. In other cases, the hybrids may not thrive as well as their parents. Nevertheless, through this interbreeding, genes may flow back and forth over the barrier--even as the barrier is becoming stronger. It may take thousands or millions of years for a new species to bud off completely from an old one. Most of these insights have emerged from studies on living species--observations on the mating habits of fruit flies, measurements of genetic divergence in wolves, and the like. (See the 2004 book Speciation if you crave the full story.) It's also possible to use these insights to learn about the origin of species in the distant past. What you need is a ton of data--such as genome sequences--from living species, and fossils to provide points of comparison. And thanks to the human genome project--as well as the chimpanzee genome project, and similar efforts to sequence DNA in gorillas, orangutans and other apes--the origin of our own species has become one of the best cases to study. In tomorrow's issue of Nature, a team of scientists from the Broad Institute in Massachusetts present the biggest such study by far. Before this study, the rough consensus among scientists was that our ancestors diverged from the ancestors of our closest relatives--chimpanzees and bonobos--at some point between five to seven million years ago. That evidence came from studies on DNA, as well as from fossils, such as the oldest hominid fossil, Sahelanthropus, which is estimated to have lived 6.5 to 7.5 million years old. Most scientists argued that the hominids made a relatively clean break from other apes, without any significant hybridizing. But that's not what has emerged from the new study. The Broad Institute scientists lined up millions of bases of DNA in humans and chimps and measured their differences. Humans and chimpanzees both inherited each segment of DNA from a common ancestor. Over time, the copies of that ancestral segment picked up mutations. The differences between them can offer clues to how long they've been evolving along separate paths. It turns out that the ancestors for some of those segments are much older than others. The only way to make sense of these results, according to the scientists, is to conclude that hominids and the ancestors of chimpanzees were interbreeding--to some extent at least--for four million years. It's hard to get a fix from the new evidence on exactly when our lineage split from the chimp lineage, and when the interbreeding finally stopped. But the authors do question whether Sahelanthropus and other early hominid fossils represent the start of a distinct line of hominids that remained pure up to living humans. They suggest that the two lineages did split apart, and hominids began to evolve the distinct hominid body (walking upright, perhaps). But later the two populations interbred, mixing their genes. So our genes have a younger ancestry than the fossil record of hominids. Some of the most intriguing signs of hybridizing comes from the X chromosome, one of the sex determining chromosomes (XX is female, XY is male). The X chromosome contains segments of DNA that have the most recent common ancestry of all the DNA shared by chimps and humans. The scientists suggest that when the ancestors of chimps and humans came back in contact a few million years ago, hybrid males turned out to be infertile--a common pattern in the origin of species among many animals. Only the females remained fertile, returning to their populations to mate and spread the genes from the other species among their own. If they had sons, those males would be fertile. As a result, many old copies of genes on the X chromosomes were lost, and the X chromosomes in today's chimps and humans share a recent ancestry. Smaller previous studies have suggested hybridization, but they've been rejected by a lot of other experts. It will be interesting to see how this one fares with the critics. I suspect it will be much harder to contest, because the sample is so vast--800 times more DNA than in previous studies. If it holds up, it raises all sorts of fascinating scientific questions. What brought the two lineages back in contact? Were they fleeing some ecological disaster? How did the two populations interact? Why didn't the two lineages collapse back into one species? Did our ancestors evolve some mating preference that turned them away for good? And how did an infusion of chimpanzee-ancestor genes alter the workings of the bodies of our ancestors? And then there is the matter of the manimals. I'm not trying to says hominids interbreeding with proto-chimpanzees are exactly the same thing as mice with human neurons growing in their heads. Obviously there's a biological difference here, because mice are separated from us by far more evolutionary time. I would just argue that any ethical stand on chimeras has to be in harmony with the scientific evidence. And it doesn't seem as if there's much careful thought behind the call for a total ban on chimeras. Integrity is not essential to a species. It emerges gradually over time, through evolutionary change. And when a new species starts to emerge, its integrity is not a foregone conclusion. Politicians may want to dismiss hybrids between animals or between plants as irrelevant to the issue of human-animal hybrids. Humans are different, the argument goes. It's certainly true that we are different from all other animals in our cognition and other faculties. And it's right to use those differences as a starting point for deciding what the ethics of new biotechnology. But when it comes to the integrity of the human species compared to other animals, there is no difference. That fact remains that our upright hominids apparently intebred with the ancestors of chimpanzees. We are all the children of chimeras. Update 5 pm: John Hawks takes issue.