Reversion, atavism, throwback are comparable when they mean return to an ancestral type or an instance of such return.
The same distinctions in implications and connotations are evident in the adjectival forms reversionary and atavistic . Reversion and reversionary are the technical terms in the biological sciences for the reappearance of an ancestral character or characters in an individual, or for an organism or individual that manifests such a character.
Atavism and atavistic are widely used both in general and in technical English. Their implication is of an apparent reversion to a remote rather than to an immediate ancestral type through the reappearance of remote, even primitive, characters after a long period of latency.
Often, in general use, this connotation of primitiveness carries with it a suggestion of barbarism or even degeneration.
Throwback is preferred to reversion or atavism by those who seek a picturesque or less technical word. It is chiefly applied to the concrete instance and is often extended to other than living things.