“Darwin’s dilemma” refers to Charles Darwin’s bafflement that the fossil record contradicted what his theory of evolution predicted. In his classic book On 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 was “a valid argument” against his theory. But he hoped that time—and more research—would provide the evidence that was lacking. Some 150 years later, the documentary Darwin’s Dilemma probes how Darwin’s dilemma has been aggravated—not resolved—by the last century of fossil discoveries, starting with the strange and wonderful creatures uncovered a century ago in the Burgess shale in British Columbia, Canada.
Darwin’s Dilemma in Darwin’s own words:
On the sudden Appearance of Groups of allied Species in the lowest known Fossiliferous Strata.
There is another and allied difficulty, which is much more serious. I allude to the manner in which many species in several of the main divisions of the animal kingdom suddenly appear in the lowest known fossiliferous rocks. Most of the arguments which have convinced me that all the existing species of the same group are descended from a single progenitor, apply with nearly equal force to the earliest known species. For instance, it cannot be doubted that all the Silurian trilobites are descended from some one crustacean, which must have lived long before the Silurian age, and which probably differed greatly from any known animal. Some of the most ancient Silurian animals, as the Nautilus, Lingula, &c., do not differ much from living species; and it cannot on our theory be supposed, that these old species were the progenitors of all the species belonging to the same groups which have subsequently appeared, for they are not in any degree intermediate in character.
Consequently, if the theory be true, it is indisputable that, before the lowest Silurian or Cambrian stratum was deposited long periods elapsed, as long as, or probably far longer than, the whole interval from the Cambrian age to the present day; and that during these vast periods the world swarmed with living creatures…
To the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods, I can give no satisfactory answer… the difficulty of assigning any good reason for the absence beneath the Upper Cambrian formations of vast piles of strata rich in fossils is very great. It does not seem probable that the most ancient beds have been quite worn away by denudation, or that their fossils have been wholly obliterated by metamorphic action, for if this had been the case we should have found only small remnants of the formations next succeeding them in age, and these would always have existed in a partially metamorphosed condition. But the descriptions which we possess of the Silurian deposits over immense territories in Russia and in North America, do not support the view, that the older a formation is, the more it has invariably suffered extreme denudation and metamorphism.
The case at present must remain inexplicable; and may be truly urged as a valid argument against the views here entertained. [emphasis added]
—Chapter IX, “On the Imperfection of the Geological Record,” On the Origin of Species, fifth edition (1869), pp. 378-381.