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What is a Common Ancestor?

October 3, 2016:

by matthewbonnanadmin |

As vertebrate paleontologists, we often want to understand the biology, life habits, and ecology of the animals preserved as time capsules in the fossil record. Beyond descriptive anatomy and metrics, fossils can often preserve valuable data at the intersection of the animal and its environment. Consider, for example, the recent revelation that some early tetrapods (Acanthostega) and placoderms (basal jawed fishes) may have segregated into juvenile and adult cohorts (Olive et al., 2016; Sanchez et al., 2016). This is not too surprising given that the larvae and juveniles of many living fish species segregate into nurseries, habitats which provide protection from predators and ample nutrients (Hoffman et al., 2015) and amphibians of the same species often separate spatially into different age classes (Ficetola et al., 2013).

Putting such discoveries into perspective, however, requires more than just convenient comparisons with living vertebrates. To best infer past behaviors, life histories, and functional morphology, the vertebrate paleontologist must understand the evolutionary relationships of their fossil organism within the context of the vertebrate family tree. Therefore, understanding the anatomy, life histories, and ecology of modern vertebrates that share a close common ancestor with the fossil of interest is essential (Witmer, 1995).

However, to the general public, the term “common ancestry” is rife with confusion. This stems in part from the continued misconceptions surrounding biological evolution. Stated plainly, biological evolution is the theory that explains the pattern of the tree of life as resulting from descent with modification from a single, common ancestor. In other words, biological evolution is the result of inherited changes passed down from one common ancestor to the next. But what is a common ancestor?

Unfortunately, in popular media evolution is often portrayed as a linear sequence from “primitive” to “advanced” life forms rather than the reality of a branching family tree. This misconception, in turn, leads to the idea that a common ancestor is a link in a chain. Although this notion of biological evolution is false, media stories surrounding fossil discoveries often continue to describe these specimens as “missing links.” Therefore, you could be forgiven for thinking that dinosaurs evolved from crocodiles, or that chickens descended from Tyrannosaurus rex (something reported as fact here: https://www.theguardian.com/science/2007/apr/13/uknews.taxonomy).

A common ancestor is not a single individual or a link in a chain, but instead represents a population of individuals from which the fossil animal of interest descended with modification. Most importantly, the living relatives of that fossil animal, no matter how closely related, have had their own evolutionary histories apart from that common ancestor and from their nearest relatives, and therefore have their own adaptations suited to their particular ecological circumstances. For example, non-avian dinosaurs such as Tyrannosaurus rex are members of the Archosauria, whose living descendants are represented today by the crocodylians and the birds (the avian dinosaurs) (Benton, 2005; Brusatte et al., 2010).

Crocodylians are more distantly related to dinosaurs than birds, but it would be a mistake to view modern crocodylians as the common ancestor to T. rex or to view birds as direct descendants of T. rex. Crocodylians and their kin parted ways with the archosaur branch that led to dinosaurs at least 240-250 million years ago, and in that time modern crocodylians evolved into semi-aquatic ambush predators (Bonnan, 2016). Therefore, Tyrannosaurus rex is not a direct descendant of a semi-aquatic predator that became terrestrial. This, in turn, means that any inferences we draw about T. rex anatomy and habits from the study of crocodylians has to be tempered with the knowledge that alligators and crocodiles have specialized traits related to their ecology that were probably not present in dinosaurs. Likewise, although birds are dinosaurs, they descended from a particular group of predatory dinosaurs, the Maniraptorans, of which T. rex is not a member (Fastovsky and Weishampel, 2012). Thus, a chicken will share some of dinosaurian traits with T. rex, but the small forelimbs of T. rex will shed no light on the origins of bird flight.

Returning to where we started, it is likely that Acanthostega, placoderms, and other early tetrapods and fishes followed life histories and survival strategies similar to those of modern fishes and amphibians. However, we cannot simply assume that what we observe in a frog or salamander was passed down unmodified from animals like Acanthostega, or that a modern bony fish is simply a changed placoderm.

Perhaps common ancestry as paleontologists perceive it is best understood through your own family tree. Although your cousins can provide insights into the likely appearance and behavior of you, you did not directly descend from your cousins, instead sharing with them a set of grandparents as your common ancestor. In other words, your cousins are not links in a family chain leading directly to you. Likewise, the living vertebrates surrounding us today retain features of common ancestors past that can help illuminate life histories and behaviors of fossil vertebrates, they are not unchanging links. Instead, they represent an amalgam of ancestral and derived (modified) features that resulted from their own unique evolutionary histories.

Submitted by: Matthew F. Bonnan, Stockton University

References Cited

Benton, M. J. 2005. Vertebrate Palaeontology, 3rd ed. Blackwell Publishing, Oxford, UK, 455 pp.

Bonnan, M. F. 2016. The Bare Bones: An Unconventional Evolutionary History of the Skeleton. Indiana University Press, Bloomington, 508 pp.

Brusatte, S. L., M. J. Benton, J. B. Desojo, and M. C. Langer. 2010. The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida). Journal of Systematic Palaeontology 8:3–47.

Fastovsky, D. E., and D. B. Weishampel. 2012. Dinosaurs: A Concise Natural History, 2nd ed. Cambridge University Press, Cambridge; New York, 423 pp.

Ficetola, G. F., R. Pennati, and R. Manenti. 2013. Spatial segregation among age classes in cave salamanders: habitat selection or social interactions? Population Ecology 55:217–226.

Hoffman, J. C., J. R. Kelly, G. S. Peterson, and A. M. Cotter. 2015. Landscape-Scale Food Webs of Fish Nursery Habitat Along a River-Coast Mixing Zone. Estuaries and Coasts 38:1335–1349.

Olive, S., G. Clément, E. B. Daeschler, and V. Dupret. 2016. Placoderm Assemblage from the Tetrapod-Bearing Locality of Strud (Belgium, Upper Famennian) Provides Evidence for a Fish Nursery. PLOS ONE 11:e0161540.

Sanchez, S., P. Tafforeau, J. A. Clack, and P. E. Ahlberg. 2016. Life history of the stem tetrapod Acanthostega revealed by synchrotron microtomography. Nature.

Witmer, L. M. 1995. The extant phylogenetic bracket and the importance of reconstructing soft tissues in fossils; pp. 19–33 in J. J. Thomason (ed.), Functional morphology in vertebrate paleontology. Cambridge University Press, USA.



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