"Clocks Versus Rocks": Fossil Explosion of Placental Mammals Contradicts "Molecular Clock" Evidence - Evolution News & Views

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"Clocks Versus Rocks": Fossil Explosion of Placental Mammals Contradicts "Molecular Clock" Evidence

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A new article in The Scientist, "Clocks Versus Rocks," reports a contradiction between the fossil record and the molecular data as regards the origin of placental mammals. The problem is that, as a fossil-based study led by Maureen O'Leary found last year, "placental mammal diversity exploded" starting around 65 million years ago, but as The Scientist now puts it, "Genetic studies that compare the DNA of living placentals suggest that our last common ancestor lived between 88 million and 117 million years ago, when the dinosaurs still ruled." So we have a conflict: fossils show the abrupt explosion of many modern mammal groups starting around 65 million years ago. However, living members of those groups are so genetically different that "molecular clock" studies suggest their origins must be deep into the Mesozoic, during the age of the dinosaurs. Which dataset are we to trust?

When evolutionary biologists hear about an "explosion," including the mammal explosion, they become nervous. Thinking under a Darwinian evolutionary mindset, they presume that mass diversity couldn't arise rapidly, and thus the fossil record is incomplete and the evolutionary origins of this group has simply not yet been discovered by fossil hunters. This, in fact, is exactly what they think is going on here, as The Scientist explains:

The researchers' main criticism is that O'Leary's team took the age of the oldest fossil from various placental groups to be the age of the group itself. This is unlikely to be true: even older fossils probably exist but have not been found yet.
Thus, molecular clock data, which places placental origins deep in Mesozoic, is preferred. Nonetheless, the fact remains that we haven't found fossils documenting the Mesozoic origin of these placental mammal groups. Rather, modern orders of placental mammals appear abruptly, in what many have called an "explosion."

Where have we heard of this problem before? The same sort of conflict exists with the "Cambrian explosion," where many animal phyla appear abruptly, without evolutionary precursors, so scientists turn to molecular data to try to push their evolutionary origins back deep into the Precambrian. Stephen Meyer covers this in detail in Chapter 5 of Darwin's Doubt. He evaluates the "molecular clock" hypothesis, which proposes that by using known mutation rates and comparing the differences between genes of living organisms, we can determine how long ago they shared a common ancestor. Molecular clock studies, however, are notoriously fraught with difficulties, and make dubious assumptions. One paper in Trends in Genetics compared molecular clock methods to "reading the entrails of chickens," 1 and another in Annual Review of Earth and Planetary Sciences admitted:

Specifically, rates of molecular evolution may vary considerably, both across taxa and over time. Furthermore the accuracy of the technique depends upon having an accurate calibration point or points, and a reliable phylogeny with correct branching order and branch-length estimates. ... The idea that there is a universal molecular clock ticking away has long since been discredited.2
Such criticisms reflect the fact that molecular clock studies have yielded widely divergent dates for the supposed most recent common ancestors of animals. As Meyer documents, some molecular clock studies place the most recent common ancestor after the Cambrian explosion and others calculate that the common ancestors of various animal groups lived prior to the origin of the universe -- obviously absurd results.3 One article acknowledges the severity of the problem:
The second area where molecules and morphology are in serious disagreement concerns the origins of the metazoan phyla. Although the difference between the molecular and morphological estimates for bird and mammal origins may be as much as 50 million years, the discord between the two for the animal phyla may be as much as 500 million years, almost the entire length of the Phanerozoic.4
In each of those cases -- birds, mammals, and the animal phyla -- we see an explosive origin of modern groups, where genetic differences between living organisms are far greater than an evolutionary view of their fossil history would suggest. Maybe that's because evolution isn't what generated their history.

References Cited:

[1.] Dan Graur and William Martin. "Reading the Entrails of Chickens: Molecular Timescales of Evolution and the Illusion of Precision," Trends in Genetics, 20 (2004): 80-86.

[2.] Andrew B. Smith and Kevin J. Peterson, "Dating the Time and origin of Major Clades," Annual Review of Earth and Planetary Sciences, 30 (2002): 65-88.

[3.] Meyer documents these studies in detail on pages 107-110 of Darwin's Doubt, and accompanying endnotes.

[4.] Andrew B. Smith and Kevin J. Peterson, "Dating the Time and origin of Major Clades," Annual Review of Earth and Planetary Sciences, Vol. 30:65-88 (2002).

Photo credit: nednerbfm/Flickr.


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