Does the Cambrian explosion challenge evolution?

The Cambrian period began about 540 million years ago, and over the following tens of millions of years the fossil record shows the rapid appearance of most major animal phyla with a fossil record, the basic body plans of animal life, including arthropods, mollusks and the first vertebrates^{[source 5]}. Darwin recognized this as a difficulty. Of the absence of earlier fossils he wrote, “The case at present must remain inexplicable; and may be truly urged as a valid argument against the views here entertained”^{[source 1]}.

In Darwin’s Doubt (2013), and earlier in a 2004 review article, Stephen Meyer argues that the explosion is not only a problem of missing fossils but of missing information: building many new body plans required new genes, new proteins and new developmental programs in a short span, and, citing published estimates of how rare functional protein sequences are, he argues that mutation and selection could not generate so much novel information so quickly^{[source 2][source 6][source 8]}. He concludes that the explosion is best explained by intelligent design^{[source 2]}.

Darwin’s own answer, given immediately after admitting the difficulty, was that earlier animals had lived but their remains had not survived: he offered a hypothesis about where the earliest fossil-bearing rocks might lie and why they would be hard to find^{[source 1]}. Meyer argues that a century and a half of searching has failed to supply the missing ancestors Darwin expected, so the difficulty Darwin acknowledged has only grown^{[source 2]}.

Meyer adds two further arguments. Animal body plans are set up early in development by networks of regulatory genes, and mutations to those early networks are almost always fatal, so he argues that the networks resist the gradual change evolution would need^{[source 2]}. And the fossil pattern runs “top down”: the big differences between body plans appear first, with the variety within each plan filling in afterward, which he argues is the opposite of what a gradual branching process would predict^{[source 2]}.

Paleontologists answer that the explosion was neither instantaneous nor without antecedents. The Ediacaran organisms lived in the tens of millions of years before it, and trace fossils record burrowing animals before the main radiation^{[source 5]}. A 2018 review of the arthropod record by Allison Daley and colleagues concluded that the group’s diversification “unfolded gradually over the ∼40 million years of the lower to middle Cambrian”^{[source 4]}.

Molecular clocks, estimates of when lineages split based on genetic differences, suggested from the 1990s that animal lineages began diverging well before the Cambrian. That prompted searches of older rocks, which have found at least 30 new Ediacaran-age lagerstätten, sites that preserve soft-bodied animals^{[source 4]}. Finding no arthropods among them, Daley and colleagues concluded that “hypotheses that regard Precambrian preservation as insufficient to preserve euarthropods can no longer be sustained”^{[source 4]}. How deep animal lineages go overall is still debated, with molecular clocks generally placing origins earlier than the fossils do^{[source 5]}.

Some Cambrian fossils do show body plans being assembled in stages. Burgess Shale-type deposits, which preserve soft tissue, record the arthropod lineage from about 518 million years ago, including stem groups such as Anomalocaris that, Daley and colleagues write, give insight into the step-by-step construction of arthropod features such as the exoskeleton, jointed limbs and segmentation^{[source 4]}.

Reviewing Darwin’s Doubt in Science, the paleontologist Charles Marshall argued that Meyer overstated how much new genetic material the explosion required: animal body plans are built largely by rewiring developmental gene networks, many of whose genes already existed before the Cambrian, rather than by inventing many new genes^{[source 3]}.

Critics also point to explanations for why the radiation happened when it did, including rising oxygen levels, new ecological relationships such as predation, and the evolution of developmental gene networks^{[source 5]}.

On the question of pace, Michael Lee, Julien Soubrier and Gregory Edgecombe estimated in 2013 that arthropod anatomy evolved about four times faster, and their genes about five and a half times faster, during the Cambrian explosion than in later periods, rates they found still consistent with evolution by natural selection^{[source 7]}.

The speed of the Cambrian radiation is real and still studied; paleontologists debate its triggers and how much of its suddenness reflects which animals could fossilize^{[source 4][source 5]}. Meyer’s conclusion, that it requires design, has not been accepted in paleontology. Proponents answer that the proposed triggers describe conditions for the radiation without explaining where the information it required came from^{[source 2]}.

The record is uneven even within the Cambrian. Trilobites, which are crown-group arthropods with mineralized skeletons that fossilize readily, appear about 521 million years ago, before some of the stem-group fossils that should precede them; many stem-group arthropods had no mineralized parts and are known only from rare deposits that preserve soft tissue^{[source 4]}. Daley and colleagues use trace fossils, such as arthropod tracks and burrows, to set limits on how long those unrecorded lineages can have existed^{[source 4]}. Proponents read the abrupt appearance of body plans as evidence that new information arose suddenly; critics read the gaps as an ordinary limit of preservation.