Deepening Darwin’s Dilemma
September 16, 2009
The newly released film “Darwin’s Dilemma” argues that the geologically abrupt appearance of the major groups of animals (the “phyla”) in the Cambrian Explosion posed a serious problem for Charles Darwin’s theory of evolution (as he himself knew), and that subsequent fossil discoveries—far from solving the problem—have made it worse.
In January 2009, however, the Journal of the Geological Society, London published an article titled “A solution to Darwin’s dilemma of 1859.” So, is Darwin’s dilemma solved, or not?
According to Darwin’s theory, all living things are modified descendants of a common ancestor. New species do not appear abruptly, but evolve from pre-existing species through a continuous series of intermediate forms. The history of life could then be represented as one “great tree,” with the universal common ancestor at its base and the many species we see today at the tips of its branches.
By 1859, geologists had discovered the broad outlines of Earth’s history in the rocks. But the fossils in those rocks did not fit the branching-tree pattern of Darwin’s theory.
In The Origin of Species, Darwin declared that if his theory of evolution were true “it is indisputable that before the lowest Cambrian stratum was deposited… the world swarmed with living creatures.” Yet Darwin admitted that the fossil record below the Cambrian strata seemed to be bereft of such creatures. Instead “species belonging to several of the main divisions of the animal kingdom suddenly appear in the lowest known fossiliferous rocks”—without any evidence of prior ancestral forms. Darwin frankly acknowledged that this lack of ancestral forms “may be truly urged as a valid argument against” his theory.
Darwin argued that fossils of the ancestors of Cambrian animals once existed but were destroyed by the heat and pressure that produced metamorphic rocks. “I look at the geological record,” he wrote, “as a history of the world imperfectly kept.” He thus concluded that the main divisions of the animal kingdom only “falsely appear to us to have been abruptly introduced” in the Cambrian.
Since 1859, however, many Precambrian fossils have been found, including microfossils of single-celled bacteria in rocks more than three billion years old. In addition, multicellular Precambrian fossils have been found in the Ediacara Hills of Australia, though there is continuing debate over whether any—or how many—of the Ediacaran fossils were animals, or what relationship—if any—they had to the Cambrian phyla. In 1998, Cambridge University paleobiologist Simon Conway Morris (who is featured in the film “Darwin’s Dilemma”) wrote, “Apart from the few Ediacaran survivors… there seems to be a sharp demarcation between the strange world of Ediacaran life and the relatively familiar Cambrian fossils” (Crucible of Creation, 30).
So there is now no shortage of Precambrian fossils. Not only do we have fossils of bacteria, but we also have many fossils of soft-bodied Multicellular organisms. “In the Ediacaran organisms there is no evidence for any skeletal hard parts,” wrote Conway Morris in 1998. “Ediacaran fossils look as if they were effectively soft-bodied” (Crucible of Creation, 28). The same is true of many of the organisms fossilized in the Cambrian explosion. The Burgess Shale, for example, includes many fossils of completely soft-bodied animals. “These remarkable fossils,” according to Conway Morris, “reveal not only their outlines but sometimes even internal organs such as the intestines or muscles” (Crucible of Creation, 2).
Yet Darwin’s excuse for the absence of innumerable Precambrian intermediates for the Cambrian phyla was that they were too small or too delicate to survive heat and pressure. The discovery of microscopic and soft-bodied Precambrian fossils makes Darwin’s excuse sound hollow; and the more such discoveries are made, the hollower it sounds.
Richard Callow and Martin Brasier reported in the January 2009 issue of the Journal of the Geological Society, London “a variety of exceptionally preserved microbes” from late Precambrian rocks in England that address “the paradox known as ‘“Darwin’s dilemma’.” At the time, ScienceDaily announced that “a solution to the puzzle which has come to be known as ‘Darwin’s dilemma’ has been uncovered by scientists at the University of Oxford,” and that “Darwin’s Dilemma” was “the lack of fossils in sediment from the Precambrian.”
But this was not Darwin’s dilemma. Darwin’s dilemma was the absence of intermediate fossils showing that the Cambrian phyla diverged from a common ancestor. Callow and Brasier didn’t solve Darwin’s dilemma. Instead, they put one more nail in the coffin of Darwin’s attempt to salvage his theory from it. The truth is that “exceptionally preserved microbes” from the late Precambrian actually deepen Darwin’s dilemma, because they suggest that if there had been ancestors to the Cambrian phyla they would have been preserved.