The human genome contains segments of genetic code from microorganisms, as well as from our ancestors, a new study has found.

In 2013, researchers announced that the genome of wasps was found to contain not just DNA from their own species, but also from microbes that live in their guts. A new study shows that a similar characteristic is also present in human beings - we carry genes from microorganisms that evolved along with our own species.

Horizontal gene transfer has been seen before in other living beings, including microbes. This process plays a role in the evolution of antibiotic-resistant bacteria. Genetic codes from microbes have also found their way into the genome of several species of animals. Some beetles are capable of digesting coffee berries thanks to genes obtained from bacteria. Nematode worms have obtained segments of DNA from microorganisms as well as plants.

The idea that horizontal gene transfer has also contributed to the genomes of humans and other complex animals has met with resistance from some researchers. If confirmed, this finding would significantly impact theories about the underlying mechanisms of evolution. Traditionally, it was believed that genes were inherited solely from parents, but the process of DNA evolution could prove far more complex than once believed. Most, if not all, animals may have evolved in part due to this transfer of foreign genes, researchers believe.

Investigators examined the genomes of human beings, 10 varieties of primates, 12 species of fruit flies and four types of nematode worms. They compared the genome seen in these organisms with other related species to determine which segments of their genome may be from foreign species.

"It's important to screen for contamination when we're doing genome sequencing, but our study shows that we shouldn't ignore the potential for bacterial sequences being a genuine part of an animal's genome originating from horizontal gene transfer," Chiara Boschetti of Cambridge University said.

Almost 150 genes in the human genome came from other species, the new study concluded. Most of these foreign genes are related to enzymes that play roles in the metabolism of amino acids and fat molecules. The ABO gene, which is responsible for determining blood types in human beings, was acquired from other vertebrates, the researchers determined.

Bacteria was found to be the most common, although not the only, source of foreign DNA. More than 50 genes from viruses were found in primates and some fungi genes were also detected in certain species.

"This means that the tree of life isn't the stereotypical tree with perfectly branching lineages. In reality, it's more like one of those Amazonian strangler figs where the roots are all tangled and crossing back across each other," Alastair Crisp, a biologist with the University of Cambridge in the UK, said.

Analysis of foreign genes in human and animal bodies was profiled in the journal Genome Biology.

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