How do people differ

What is the difference between humans and monkeys?

About five to six million years ago there was a primate species in Africa from which both humans and the two chimpanzee species known today, the dwarf chimpanzee and the common chimpanzee, developed. Since all other primate species that arose during that time are now extinct, we rightly call the chimpanzees "our closest relatives". But what changes have occurred in the primate genes so that two species as physically and mentally as different as chimpanzees and humans could develop?

Already in the 18th century Linnaeus recognized the similarity between humans and chimpanzees and assigned them both to primates, but - in order to keep a proper distance - into different families. But it was only 25 years ago that scientists discovered that at the molecular level, for example when comparing the genetic makeup of humans and chimpanzees, the similarity is actually a striking 99 percent. This means that the genetic match between humans and chimpanzees is in a range that normally applies to subspecies of a species and not to two independent families. Because even within one species there can be differences of this magnitude: Two fruit flies also differ in an average of one percent of their genetic make-up.

More recent analyzes of the Leipziger Max Planck Institute for Evolutionary Anthropology confirm the originally rough estimate of the molecular similarity between humans and chimpanzees: Svante Pääbo's research team sequenced around three million base pairs from the chimpanzee genome. Compared to the corresponding sections in the human genome, an average of 987 out of 1000 bases - i.e. 98.7 percent - were identical. Therefore, the remaining 1.3 percent difference must be the biological basis for what makes a human human and what distinguishes it from a chimpanzee. Wolfgang Enard, the first author of the study, says: "1.3 percent difference doesn't sound like much, but it adds up to a haystack of 39 million possible differences. Finding the few differences that actually make a difference is the real big one Challenge."

The scientists were particularly interested in the basics of our cognitive abilities, especially language skills. But there are also important medically relevant differences: These include the resistance of chimpanzees to AIDS. There is also evidence that diseases such as breast, colon or lung cancer occur much less frequently in chimpanzees than in humans. Furthermore, Alzheimer's disease or malaria are spread differently in humans and chimpanzees.

In a first step, the Max Planck scientists analyzed differences in the expression of genes in humans and chimpanzees or - to put it more generally - studied the evolution of the human "transcriptome". The regulation of the transcription of genes, i.e. which gene is copied at what point in time and in what amount, is an important mechanism in biology. A copy of a gene is generated, which in turn serves as a template for the synthesis of a corresponding protein. The entirety of all genes transcribed in a cell or a cell type at a certain point in time is called - in analogy to the "genome" - also "transcriptome".

For their analysis, the scientists used so-called gene chips, which contained up to 18,000 human genes on a nylon or glass carrier surface. This enabled them to investigate how many copies of the genes there are in a tissue or cell type, i.e. how often the gene is used. The researchers marked the copies of the genes from a certain tissue radioactively or with a fluorescent dye and then placed them on the glass or nylon carrier. Since the marked copies and the genes on the carrier were complementary to each other, i.e. like a key and lock, a marked copy only bound where the corresponding gene was located and gave a radioactive or fluorescent signal at this point. The strength of the signal was then determined from the number of copies of a gene present. By performing this process on different tissues one after the other - for example with a human and then with a chimpanzee liver - the scientists were able to identify the genes that differ between these types in the liver.

In this way, the researchers found that there are a significant number of genes that are used differently in humans and chimpanzees. In the course of evolution, the use of genes in the human brain seems to have changed much more than in the chimpanzee: Almost four times as many differences have accumulated in the human brain. This is a first important indication that the functional differences between a human brain and that of a chimpanzee also find a correspondence at the molecular level. Svante Pääbo, the head of the project, says: "At first we weren't sure we'd find even one significant difference. Now we have hundreds."