Many of the illnesses we suffer today are down to our ancestors not having enough choice in the mating game, UK researchers believe.
Early man had fewer mates to choose from
Inbreeding over the millennia has left us with "sloppy" control over our genes, making us vulnerable to disease.
Had there been a greater choice of mates, natural selection would have removed these harmful gene mutations, the Bath University team says.
Their findings appear in the journal Public Library of Science Biology.
They looked at how the DNA of humans, monkeys, rats and mice had evolved.
They found key regions of our DNA, which control the switching on and off of genes, have been altered by around 140,000 naturally occurring mutations over the last six million years.
Compared with the DNA of rats and mice, human and chimpanzee DNA was much less carefully controlled.
The researchers believe that most of the damaging mutations occurred when there was only a small population of early hominids - the two-legged primates who later evolved into humans and chimpanzees.
At the time, there may have been as few as 10,000 hominids to breed with one another.
In comparison, rats and mice had plenty of mates to choose from and harmful DNA mutations were rapidly eliminated from the gene pool, according to the study authors from the Universities of Bath, Edinburgh and Sussex.
Dr Martin Lercher, a co-researcher, said: "We are used to viewing us as the pinnacle of evolution, but seeing that rodents control their genes much more precisely is somewhat sobering."
Lead investigator Professor Peter Keightley, from Edinburgh University, said that although humans now had many more mates to choose from, it was unlikely that the harmful mutations would be eliminated in the near future.
However, he said each mutation had only a small effect and it was likely that "positive" mutations had also been selected during human evolution that balanced out some of the harmful ones.
He said the effects were more of a concern for our nearest primate relatives - the chimpanzees, gorillas and orang-utans - because accumulation of harmful mutations was more of an issue in small captive populations.
"They are in imminent danger of becoming extinct," he said.
Chris Tyler-Smith, at the Wellcome Trust Sanger Institute, said: "We know that population sizes in the past were very small.
"We also know that natural selection is less efficient in small populations.
"But this work is the first to spell out the consequences: less precise control of gene expression in humans. It shows how useful genome sequences can be."