When it comes to our DNA, it's well known that we inherit it from our parents, and they inherit it from their parents, and so on and so forth. But now new research suggests that humans may have genes that are not passed down from our ancestors.

So where do these other genes come from? In a process known as horizontal gene transfer (HGT), certain organisms acquire essential "foreign" genes from microorganisms living alongside them in the same environment. While this phenomenon has been largely seen in single-celled organisms, and is thought to play an important role in the evolution of some animals including nematode worms and some beetles, this study is the first to show that HGT occurs in humans.

The idea that more complex animals can undergo HGT has been widely debated. But now new findings published in the journal Genome Biology show that even humans acquire tens or hundreds of active "foreign" genes from this process.

"Surprisingly, far from being a rare occurrence, it appears that HGT has contributed to the evolution of many, perhaps all, animals and that the process is ongoing, meaning that we may need to re-evaluate how we think about evolution," lead author Alastair Crisp from the University of Cambridge, UK, said in a press release.

The researchers studied the genomes of 12 species of Drosophila or fruit fly, four species of nematode worm, and 10 species of primate, including humans. In order to determine whether they were foreign or simply passed down, they calculated how well each of their genes aligns to similar genes in other species.

What they found were 128 previously unknown foreign genes in the human genome acquired by vertebrates via HGT, including some which were involved in metabolism, immune and antimicrobial responses, and even coding for the ABO blood group. But where did these foreign genes come from?

According to the researchers, the majority of these genes originated from bacteria and protists, while others came from fungi and viruses - which accounted for up to 50 percent more foreign genes in primates.

Until now, previous research only focused on bacteria as a source of HGT, so the idea that humans could possess foreign genes was rejected. But now, this study shows that scientists had underestimated the true extent of HGT in complex animals such as humans.

The findings could potentially impact any future genome sequencing, during which researchers usually remove bacterial sequences because they are seen as contaminants. However, it now appears that they an important part of the human genome, and should be taken into consideration.

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