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Even among pre-humans, interbreeding with different species has left its marks in our DNAs including Neanderthals and Denisovans. So species "kinds" lines are not as sharply drawn in nature as our author here suggests.

And then there are the retroviruses that have inserted themselves into human (and other mammalian) DNA throughout evolution and become a permanent part of our genome. A sizeable fraction of our DNA is, in fact, viral DNA. One fascinating consequence of the viral DNA contained within our genome is that a protein essential for placenta formation is, in fact, viral in origin. Which begs the question, if it were not for viruses inserting themselves into the mammalian genome tens of millions of years ago, would mammals carry around some sort of egg inside, and give birth only when the egg hatches? Some fish reproduce in that manner.

At this time, there is a retrovirus spreading among koalas. The indication is that the retrovirus is making itself a permanent part of their genome.

The process of evolution is truly amazing.

This article came out when I was in Grad school and it was a game-changer for me.

Nature. 1987 Sep 3-9;329(6134):75-9.
An intronless gene encoding a potential member of the family of receptors coupled to guanine nucleotide regulatory proteins.

Kobilka BK, Frielle T, Collins S, Yang-Feng T, Kobilka TS, Francke U, Lefkowitz RJ, Caron MG.
Abstract
Plasma membrane receptors for hormones, drugs, neurotransmitters and sensory stimuli are coupled to guanine nucleotide regulatory proteins. Recent cloning of the genes and/or cDNAs for several of these receptors including the visual pigment rhodopsin, the adenylate-cyclase stimulatory beta-adrenergic receptor and two subtypes of muscarinic cholinergic receptors has suggested that these are homologous proteins with several conserved structural and functional features. Whereas the rhodopsin gene consists of five exons interrupted by four introns, surprisingly the human and hamster beta-adrenergic receptor genes contain no introns in either their coding or untranslated sequences. We have cloned and sequenced a DNA fragment in the human genome which cross-hybridizes with a full-length beta 2-adrenergic receptor probe at reduced stringency. Like the beta 2-adrenergic receptor this gene appears to be intronless, containing an uninterrupted long open reading frame which encodes a putative protein with all the expected structural features of a G-protein-coupled receptor.

https://www.ncbi.nlm.nih.gov/pubmed/3041227

Lefkowitz has since won the Nobel Prize, quite deservedly.