Can humans and chimpanzees produce offspring

Schoolchildren have always suspected it: Occasionally, what they know in school does not match real life. To explain what a species is, biology books often use the example of the horse and donkey. When these mate with each other, the offspring - mules or mules - are always sterile. It is concluded that only parents of one species would jointly produce offspring that can continue to reproduce. "When we have teachers visiting here, it is always difficult to talk them out of this term," says Dietmar Zinner from the German Primate Center in Göttingen. Last week, biologists met there to discuss the not so rare exceptions to the textbook rule: the exchange of genes across species boundaries.

Butterflies do it, fish, birds, and also many monkeys: They not only have sex with partners of other species, but also produce fertile offspring in the process. In this way, genetic material can switch from one species to another - scientists speak of hybridization. Occasionally, when genes cross-link, new species even emerge.

So in evolution there are not only branches, but also connections. One should therefore not speak of the tree, but of the web of life, says evolutionary researcher Michael Arnold from the University of Georgia in Athens, USA. Some researchers claim that this could even be important for the origin of humans.

The fact that monkeys do not always remain true to their species was initially only noticed by chance, reports Zinner. For behavioral studies it was necessary to determine exactly where which species live. The researchers used genetic methods to differentiate the species, which are often quite similar. The basic idea behind it: The more random mutations differentiate between two species, the longer it has been since they separated in evolution, i.e. the more distant the relationship. The results were confusing, however. Depending on which sections of the DNA genome molecules the biologists analyzed, they came to different relationships.

In Africa, for example, there are five types of baboon. But if you do not look at the genes in the cell nucleus, which among other things determine the appearance, but rather the genetic material in the mitochondria, the power plants of the cell, you get a lot more groups. On the one hand, animals were found with the same mitochondrial genome belonging to different species, on the other hand, there are groups within one species with different mitochondrial DNA. The most plausible explanation is cross-species crossbreeding, says Zinner. All baboon species can eventually mate with one another. "The more molecular methods we use, the more hybrids we discover."

Hybrids are not machos

Mostly it is male animals that mate with females of a neighboring species, adds Christian Roos, a colleague of Zinner's geneticist. The foreign genome affects the behavior of the animals. There are baboons, in which the females determine, and other species, whose males quite brutally hold together a harem of females. Hybrids of the two are then less macho in their demeanor than animals without the sprinkles in the genetic make-up. In baboons, there must have been multiple splits and re-encounters between the species, Roos and Zinner assume based on genetic data.

If two closely related animal species inhabit neighboring habitats, species-overlapping techtelmechtel are particularly common in the border area. For example with the mouse lemurs in Madagascar: the gray-brown mouse lemur lives in dry forest areas, the gray mouse lemur in damp coastal forests. The Makis cross where the two habitats meet. If these hybrids can cope better with life in the transition zone, a new species could emerge from them, explains Dietmar Zinner.

The stubby-tailed macaque, native to Southeast Asia, probably owes its existence to a similar coincidence. Genetic analyzes indicate that this monkey species, which is related to the monkeys, emerged in evolution from a union of long-tailed macaques with a common ancestor of today's Assam macaques and the Tibetan bear macaques.

Whether such connections in the primate network also played a role in human evolution is currently a hotly debated topic among researchers. There is some evidence that after the split between the ancestors of humans and the ancestors of the chimpanzees, there was still sexual contact between the two lines. A comparison of much of the genome suggests that the two lines split more than six million years ago; but there are also some genes that indicate a later separation. Nick Patterson from the Massachusetts Institute of Technology in Cambridge, USA, therefore thinks it is conceivable that the two groups might have tied up with each other again before they finally went their separate ways.

However, there are other explanations. Graham McVicker from the University of Washington in Seattle recently showed that the differences can also be due to the fact that beneficial genes are particularly well preserved and less beneficial ones are sorted out. In order to eliminate such influences, researchers actually often use areas for time measurement in the genome that do not serve as building instructions for proteins, where mutations have neither advantages nor disadvantages. McVicker found, however, that evolution also prefers to conserve those sections that are only in the vicinity of beneficial genes. In these areas, the genetic clock may tick differently than researchers previously believed.

Mosaic of human genes and monkey genes

Michael Arnold is convinced, however, that hybridization is an important mechanism of evolution - also in the development of Homo sapiens. After all, the human genome is a mosaic of modern genes and the genome of his ape ancestors. In some parts of the DNA, for example, humans are more similar to the gorilla than to their recognized closest relative, the chimpanzee. But that doesn't necessarily mean that the ancestors of humans and gorillas had sex with each other. The cause of the similarities is possibly a mechanism that the experts call "incomplete lineage sorting", explains Johannes Krause from the Max Planck Institute for Evolutionary Anthropology (EVA) in Leipzig: When the common ancestors of all humans and great apes have different variants of a gene at the same time existed, it may well be that when the lines were split, one variant happened to survive in humans and gorillas, while the chimpanzee got a different shape.

The old question of whether there were tidbits between the ancestors of modern man and other early human species also remains undecided. Arnold thinks such liaisons are very likely. He suspects, for example, that our ancestors cuddled with Homo erectus and thereby contracted a type of lice that arose long before Homo sapiens and until then only plagued the elderly.

Modern humans are said to have inherited something more gratifying from the Neanderthals: A gene variant that promotes brain size appeared in Homo sapiens at the same time that Homo sapiens divided the Neanderthals' settlement area around 40,000 years ago. However, the results of the EVA researchers do not confirm these speculations. Johannes Krause, who works on the sequencing of the Neanderthal genome, states: "Neither in the mitochondria nor on the Y chromosome did we find any evidence of a gene exchange between Neanderthals and Homo sapiens." He does not want to rule out the occasional affair one hundred percent, but there is no evidence. The sequencing of the Neanderthal genome should be completed later this year. It will certainly provide new arguments for the old dispute.