Complex sentence: The human genome is a living database of ancient and extinct viruses that once infected hominids and primates and have left copies of their DNA in the DNA of modern humans; about 31 different families of viruses, called human endogenous retroviruses, have been discovered.
Split into simpler sentences: The human genome is a living database of ancient and extinct viruses that once infected hominids and primates. These viruses have left copies of their DNA in the DNA of modern humans, and about 31 different families of viruses, called human endogenous retroviruses, have been discovered.