3.3: Fossils and family relationships- introducing cladistics

Biofundamentals 2e · LibreTexts

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    Fossils provide evidence for the existence, in the past, of both familiar and previously unknown "types" of organisms. An important point to keep in mind is that organisms differ in their fossilizability, which depends upon the environments in which they lived. Most types of organisms have left no fossils whatsoever.[96] That said, fossils provide critical evidence for understanding the history of life. An obvious question, answered through the study of fossils, is whether Ediacaran organisms[97], Burgess shale creatures (→), and dinosaurs fit into the same classification scheme as living organisms or do they form separate family trees? Do they provide evidence for multiple independent origins of life. The result of many detailed studies lead to the conclusion that essentially all fossilized organisms fall into single classification scheme. A classic example are dinosaurs which are clearly related to a specific type of reptile and gave rise to modern birds, while mammals are more closely related to a second, now extinct, group of “mammal-like reptiles.” If we had samples of Ediacaran organisms we could quickly (through DNA analyses) resolve how they are related to modern organisms.[98] It has been possible to extract and analyze DNA from the bones of Neanderthals and Denisovian-type humanoids; both species went extinct ~30,000 years ago. Analysis of this DNA has clarified the relationship between Neanderthals, Denisovians, and modern humans [99] providing compelling evidence for inbreeding between these groups. The result is to reclassify Neanderthals and Denisovians as subspecies of Homo sapiens.[100]

    Questions to answer:

    1. Explain why extinct species fit into the same classification scheme as used for living (observable) organisms.
    2. What factors would influence your decision on whether a trait found in two different organisms was present in their common ancestor?

    Questions to ponder:

    • You discover life on a planet orbiting another star in another galaxy; would you expect such organisms to fit into the Linnaean classification system?
    • Would sex with a Neanderthal be immoral?

    The theory of evolution and the organization of life

    Why is it that pterosaurs, birds, whales, and humans share common features, such as the organization of their skeletons, similarities that lead them to be classified together as vertebrates? Why are there extinct organisms, known only from their fossils, but which nevertheless share many common features with living organisms? And most importantly, why are there so many different types of organisms? Charles Darwin (1809-1882) and Alfred Wallace (1823-1913) proposed a model, described in detail in Darwin’s book "On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life" originally published in 1858, and more succinctly by Wallace, that answered these and a number of other questions.

    The main unifying idea in biology in Darwin's theory of evolution through natural selection

    - John Maynard Smith

    Evolutionary theory is based on observations of the natural world and their logical implications. Organisms are similar because they are related to one another – they share common ancestors.[101] We can infer that the more characters two species share the more recently they shared a common ancestor. We can make plausible and testable deductions about what those common ancestors looked like. As an example, we predict that the common ancestor of all terrestrial vertebrates resembles a fish with leg-like limbs and we can predict the number and shape of the bones found in those limbs.[102] Scientists have discovered fossils of such an organism (← Tiktaalik roseae).[103]

    Illustration of a slender, elongated creature resembling an amphibian, with a smooth body and distinct head.
    Illustration of a slender, elongated creature resembling an amphibian, with a smooth body and distinct head.

    Tiktaalik roseae, an extinct organism that lived ~375 million years ago, is likely to be similar to the common ancestor of all terrestrial vertebrates (from “Your inner fish” by Neil Shubin).

    The discovery of Tiktaalik and other "missing" links is an example of the fact,well before the molecular, cellular, and developmental mechanisms were uncovered evolutionary theory "made sense" of what was observed, made testable predictions about what would be found, and where to look, and has been supported by what has, in fact, been found. In the case of particularly fast growing organisms, and very strong selection pressures, such as the presence of an antibiotic, we can observe evolutionary processes taking place over the course of days, weeks, and months – that is, in real time.[104]

    This page titled 3.3: Fossils and family relationships- introducing cladistics was last modified on Thu, 08 Jan 2026 23:33:54 GMT and is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Michael W. Klymkowsky and Melanie M. Cooper.