Can Fossils Indicate Ancestry?

Post Author: Bill Pratt

In most cases, fossils cannot give us ancestry, according to paleontologist Donald Prothero.  This statement, coming from Prothero, shocked me when I first read it, because it seems like fossil news headlines always make claims about ancestry, but here is Prothero, a staunch supporter of evolution, disagreeing.

Let me explain, lest I be accused of quote mining.  In his book Evolution, Prothero dedicates an entire chapter to explaining how scientists classify plant and animal life.  According to Prothero, the dominant method used today is cladistics, where the relationships among animals and plants are determined by the comparison of shared derived characters.  This theory has only taken hold in the last few decades, replacing older systems of classification.

A cladogram (cladistic diagram) comparing an assortment of vertebrates (e.g., lamprey, shark, frog, cow, monkey, human) might look at shared derived characters such as jaws, vertebrae, lungs, four legs, hair, mammary glands, opposable thumb, and stereovision.  Cladograms are powerful tools for classifying life because they are using directly observable evidence taken from living animals and from fossils.  But do cladograms indicate fossil ancestry?  Only minimally.  Here is Prothero:

Some aspects of cladistic theory have proven more difficult for many scientists to accept.  For example, a cladogram is simply a branching diagram of relationships between three or more taxa.  It does not specify whether one taxon is ancestral to another; it only shows the topology of their relationships as established by shared derived characters.  In its simplicity and lack of additional assumptions, it is beautifully testable and falsifiable.

Prothero explains that cladistics frustrate some evolutionists who want to say more about ancestry from the cladograms, but Prothero urges caution:

The biggest sticking point is the concept of ancestry.  We tend to use the term ancestor to describe certain fossils, but we must be careful when making that statement.  If we want to be rigorous and stick to testable hypotheses, it is hard to support the statement that ‘this particular fossil is the ancestor of all later fossils of its group’ because we usually can’t test that hypothesis.  Because the fossil record is so incomplete, it is highly unlikely that any particular fossil in our collections is the remains of the actual ancestor of another taxon.

What is refreshing about these statements from Prothero is that we are seeing actual scientific restraint when it comes to the analysis of fossils.  Unfortunately this kind of restraint is almost never present when the news media trumpet a new fossil find.  We only hear about “missing links” and how “X is the ancestor of Y” throughout the reports.

To be fair to Prothero, he does believe that ancestry can be verified if the fossil sample size is large enough.  In his own research on planktonic microfossils, he claims that there are enough layers of fossils to draw scientific conclusions about ancestry.

Planktonic microfossils aside, it is time that palentologists become more careful with their language and stop referring to new fossils in sensational terms.  In most cases, there is no way to determine ancestry; we can look at what features a new fossil shares with other taxa, but that is usually as far as we can go.

  • Andrew Ryan

    Interesting, here’s what your friend Jerry Coyne says about the book:

    “What the Fossils Say is simply the best existing book on the evidence for evolution from fossils; there is no competitor. It lays out, for the educated layperson, not only the process of fossilization and the biases it imposes on our knowledge of ancient life, but also discusses systematics, how fossils are used to infer genealogy, and (my favorite part), the evidence. All the favorite transitional forms are there, in nice detail, and many that are not as well known. Prothero does not shy away from engaging creationists – he uses the evidence against their wrongheaded claims at every turn.”

    Your interpretations of the book are somewhat at odds! Out of interest, have you read the whole book?

  • Bill Pratt

    Yes, I read the whole book. How is my post mis-interpreting Prothero?

  • TheMuse

    Fossils can’t definitively give us ancestry. It can provide hints and important clues though. And while fossil evidence may not provide definitive evidence, DNA evidence certainly does. I am now reading reading Relics of Eden (The powerful evidence of evolution in Human DNA) by Daniel Fairbanks and it is fascinating and compelling. Fairbanks by the way is deeply religious(he is a Mormon) for anyone ‘worried’ about that sort of thing.