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NotSanguine writes:

It has long been known that microbes, as well as less complex multicellular organisms, often transfer genetic information between them and incorporate it into their own genomes.

A study, published on March 13, 2015 in the journal Genome Biology has identified a number of genes which have been transferred to complex animals, including humans. From the Phys.Org writeup on the study:

The transfer of genes between organisms living in the same environment is known as horizontal gene transfer (HGT). It is well known in single-celled organisms and thought to be an important process that explains how quickly bacteria evolve, for example, resistance to antibiotics.
...
Lead author Alastair Crisp from the University of Cambridge, UK, said: "This is the first study to show how widely horizontal gene transfer (HGT) occurs in animals, including humans, giving rise to tens or hundreds of active 'foreign' genes. 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."
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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. They calculated how well each of their genes aligns to similar genes in other species to estimate how likely they were to be foreign in origin. By comparing with other groups of species, they were able to estimate how long ago the genes were likely to have been acquired.

A number of genes, including the ABO blood group gene, were confirmed as having been acquired by vertebrates through HGT. The majority of the other genes were related to enzymes involved in metabolism.

In humans, they confirmed 17 previously-reported genes acquired from HGT, and identified 128 additional foreign genes in the human genome that have not previously been reported.

It would be quite interesting to discover whether or not antibiotics and the western obsession with disinfectants has slowed this process or not.