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Fossil Friday: Direct Fossil Ancestors of Living Species?

March 8, 2024, 6:44 AM

This Fossil Friday we look into a question that relates to two previous posts about gradual species-to-species transitions (Bechly 2019) and chronospecies (Bechly 2024). It is the often posed obvious question of whether there are any known fossil species that are believed to be the direct ancestors of living species, thus not just cousins on side branches of the stem group but actual stem species on the stem lineage.

Given that there are millions of living species and each of them must have had numerous successive ancestral species in their stem lineage, we might expect to find a lot of examples in the technical literature. However, actually there are only very few cases, where such direct ancestor-descendant relationships have been proposed. All these cases are only weakly supported and not uncontroversial. They are generally restricted to groups with a rich Plio-Pleistocene fossil record.

A Few Assumed Examples

Among the few examples for mammalian species would be the extinct European canids Canis etruscus (featured above) and Canis mosbachensis as assumed ancestors of modern wolves (https://en.wikipedia.org/wiki/Evolution_of_the_wolf), or the extinct Bison antiquus and Bison occidentalis as assumed ancestors of the modern American bison (https://en.wikipedia.org/wiki/Bison#Evolution_and_genetic_history), or the extinct Elephas hysudricus as ancestor of the modern Asian elephant (https://en.wikipedia.org/wiki/Asian_elephant#Evolution). Another example might be the steppe mammoth Mammuthus trogontherii as ancestor of the woolly mammoth Mammuthus primigenius (https://en.wikipedia.org/wiki/Woolly_mammoth#Evolution), which only went extinct about 4,000 years ago on Wrangel Island and thus may count as modern species. Some paleoanthropologists suggested that the Eurasian archaic human Homo heidelbergensis was via Homo steinheimensis the direct ancestor of Neanderthals, and the African archaic human Homo rhodesiensis could be the direct ancestor of later Homo sapiens in north-eastern Africa, but this is quite controversial, as is the valid taxonomic status of these fossil human species themselves.

More examples for assumed direct ancestors of living species are found in Pliocene and Pleistocene marine protists among the foraminiferans and radiolarians (see Prothero & Lazarus 1980 and Lazarus 1983), mainly because we have a long-lasting undisturbed microfossil record of these unicellular organisms in deep sea sediments. But of course, this raises the fuzzy question of chronospecies (Bechly 2024) and whether we are only seeing microevolution within a species rather than speciation (which arguably could be an arbitrary distinction).

Slim Evidence

Overall, it looks like the evidence for direct fossil ancestors is extremely slim to say the least. Why is that? Of course, one possibility would be that common descent is wrong, but even young earth creationists affirm that speciation does occur and that wolves or bisons had ancestral species respectively. So, could there be another explanation?

Indeed, there are two general problems concerning the recognition of direct ancestors in the fossil record:

1.) Even though the fossil record is pretty complete on the macroevolutionary level of higher taxa (family and above), the fossil record will always be highly incomplete on the lower taxonomic levels because it was estimated that less than 1% of all species were preserved as fossils (not to speak of having been discovered). This seems to make it very unlikely to find exactly the direct ancestor of a particular living species. However, renowned expert Mike Foote (1996) estimated the probability of ancestors in the fossil record and found the probability to find ancestor-descendant pairs not negligible and likely underestimated.

2.) There is a general methodological impossibility to diagnose a direct ancestor species based on morphological characters, because all of its primitive characters are shared with other stem group representatives and outgroup taxa, while all of its derived characters are inherited by its descendant species. There is no diagnostic character that would label an ancestor as such. We should expect an ancestor to lack any autapomorphies that would indicate its position on a side branch, but given the incomplete preservation of fossils we can never know if such characters were just not preserved or even have not been preservable at all (e.g., soft tissue, genetic, behavioral, physiological characters). The only theoretical case where a stem species could possess a unique diagnostic character, would be the esoteric case when it developed a derived trait that then immediately got lost in the descendent species. However I don’t know of a single example, and such a case would also not be empirically distinguishable from a side branch autapomorphy. So overall, fossil ancestors can in principle not be recognized as such based on their characters. Therefore, there can always only be plausibility arguments, based on a continuous preservation and certain criteria such as occurrence older but in the same region as the descendant species (Prothero & Lazarus 1980).

A Very Hard Task

Because of such problems, Professor Willi Hennig, the founder of phylogenetic systematics (cladistics), recognized that finding and demonstrating direct ancestors would be a very hard task. British paleontologist Colin Patterson even “viewed cladistics as a corrective to the deeply problematic practice of identifying individual fossils as direct ancestors of more recent groups” (Nelson 2000 quoted in Quinn 2017). Therefore, modern paleobiologists generally do not even look for ancestor-descendant relationships but only for sister group relationships in terms of more recent common descent.

So, even if Darwinism were true, we would not expect to find a lot of direct fossil ancestors of living species. But on the other hand, there is certainly no positive support for Darwinism from such elusive ancestors either. I would consider this to be a draw.

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