You are here
The Sequence of Transitional Fossils
Do transitional fossils appear in sequence as they should? problem
Summary of problems with claim:
Paleontologists employ methods to test whether their results are better than would be expected by chance. Evolutionary biologists draw data from multiple lines of evidence, and each of those lines of evidence reveals the same pattern — the same branching tree of life.
Explore Evolution asserts:
Given the millions of different fossil forms in the fossil record, critics argue that we would expect to find, if only by pure chance, at least a few fossil forms that could be arranged in plausible evolutionary sequences. To understand what they mean, imagine that a representative of every organism that has ever lived on earth was randomly pasted to an enormous wall representing the geologic column. Most of the fossils would bear no relationship to the other fossils stuck closest to them on the wall. Nevertheless, by chance a few of them might end up next to forms that do have some resemblance. These forms might then appear to be related as ancestors and descendants, even if they were not. Is it possible that the mammal-like reptile sequence is a statistical anomaly rather than a legitimate sequence of ancestors and descendants.
Paleontology doesn't work in a vacuum. In other words, paleontology is only one branch (or subset) of evolutionary biology. Paleontological contributions consist of morphology and stratigraphic sequence data, data that is then integrated with morphological data from living critters, their DNA (and other molecules), as well as developmental data that all work together to support common descent.
In other words, as DNA changes, that affects the proteins produced, the developmental trajectory, and the subsequent morphology of traits in individuals and lineages through time. All of these fields – molecular biology, developmental biology, and comparative anatomy – are intertwined at the heart of evolutionary theory. These fields each work from independent data sets, yet still return to converge on similar answers. For example, molecular sequence data tells us that humans and rats share more DNA in common than either do with a bird; that birds, humans, and rats are more genetically similar than any are to salamanders; and that salamanders, birds, rats, and humans share more DNA with each other than any do with salmon. In turn, developmental and morphological features are shared uniquely with this same pattern of similarity and dissimilarity. For example, humans and rats have placentas (as embryos) and hair as adults while birds do not. Birds, rats, and humans possess a chorion and allantois (extra-embryonic membranes – modified in mammals, though still present) during development, and a fully developed atlas/axis (vertebral arches) that allow up/down and side-to-side movement of the head – features which salamanders do not have. Finally, salamanders, birds, humans, and rats all share unique modes of digit development and adult hips that are fused to the vertebral column – features lacking in salmon. Thus, evolutionary biologists do not solely rely on fossils to understand evolutionary relationships; instead, fossils fall into a much larger picture of other data that returns to corroborate our understanding of history.
The statistical argument presented by Explore Evolution not only fails from a strict paleontological perspective – since paleontologists test for randomness in their data sets – but moreover, paleontologists work within the bracket of extant diversity, and data from living critters also contributes to our understanding of relationships and ancestral/derived traits.
A recent paper addressing this evidence explained:
It is clear that the fossil record cannot be read literally (Darwin 1859). There are many gaps, and many organisms, and indeed whole groups of poorly preservable organisms that have never been preserved and are doubtless lost for ever (Raup 1972). Some have even gone so far as to suggest that the fossil record is almost entirely an artifact of the rock record, with appearances and disappearances of fossil taxa controlled by the occurrence of suitable rock units for their preservation (Peters and Foote 2001, 2002), or the matching rock and fossil records controlled by a third common cause (Peters 2005). However, the widespread congruence between the order of fossils in the rocks and the order of nodes in cladograms (Norell and Novacek 1992; Benton et al. 2000) indicates that the order of appearance of lineages within the fossil record is not a random pattern.
The Norell and Novacek (1992) citation is the same source Explore Evolution cites to justify the claim that the fossil record is statistically problematic. How can this conflict exist? The problem comes from two sources. First, that the authors of Explore Evolution do not understand (and count on their audience not understanding) how fossils are actually used. As Benton and Donoghue observe, "Fossils can provide good 'minimum' age estimates for branches in the tree, but 'maximum' constraints on those ages are poorer." In order to find a fossil possessing a transitional feature, it is necessary for that feature to have evolved, for the population in which it evolved to diversify, and for some descendant of the first individual with that trait to have died, been fossilized, and that a paleontologist have discovered that fossil. More intense sampling will never move the oldest date of a feature closer to the present, but will move that earliest occurrence back further, approaching the time of its first occurrence.
A second error derives from the authors' fundamental misunderstanding of evolutionary processes. Lineages with traits characteristic of a transitional form may persist long after another lineage has evolved novel traits, and which lineage will have the oldest fossil will depend on where and how fossils from each group formed, and where paleontologists have looked for those fossils. Complaints that such fossils are not in sequence are equivalent to claiming that my grandmother could not be ancestor because she and I lived at the same time. The figure illustrating mammalian evolution in the section below demonstrates how the overlapping histories of different lineages could produce fossils which appear to be out of order if the branching evolutionary process were not clear.
This explains why Norrell and Novacek, after observing that the fossil record of primates is spotty because the sequence of the earliest representatives (as of 1992) of two groups is not as predicted (because the fossil record is limited or absent for those groups), nonetheless state, "Despite these discrepancies, there is a noteworthy correspondence between the fossil record and the independently constructed phylogeny for many vertebrate groups. Statistically significant correlations were found in 18 of 24 cases examined" (Mark A. Norrell and Michael J. Novacek (1992) "The fossil record and evolution: comparing cladistic and paleontological evidence for vertebrate history," Science 255(5052):1690-1693). It is worth noting that new fossil discoveries and improved phylogenetic reconstructions in the 15 years since they wrote that paper, have resulted in a much improved fit between hypothesis and the hominid fossil record. This illustrates the danger in basing an argument on what we don't know, the core of the argument of Explore Evolution.