In Explaining the Cambrian Explosion, Has the TalkOrigins Archive Resolved Darwin's Dilemma?


A correspondent recently referred me to an article in the TalkOrigins Archive responding to the argument that "Complex life forms appear suddenly in the Cambrian explosion, with no ancestral fossils." TalkOrigins is a popular online resource that collects attempted answers to some often-heard challenges to Darwinian evolutionary theory. The article offers seven responses to the contention that the Cambrian explosion, which occurred some 530 million years ago, represents a significant difficulty for the neo-Darwinian view on how animal body plans evolved.

Since this subject comes up frequently in the evolution debate, as indeed the seeming dilemma posed by the Cambrian event troubled Darwin himself, I here offer a brief reply to TalkOrigins.

Response #1: Complex life preceded the Cambrian.

The article explains,

The Cambrian explosion was the seemingly sudden appearance of a variety of complex animals about 540 million years ago (Mya), but it was not the origin of complex life. Evidence of multicellular life from about 590 and 560 Mya appears in the Doushantuo Formation in China (Chen et al. 2000, 2004), and diverse fossil forms occurred before 555 Mya (Martin et al. 2000). (The Cambrian began 543 Mya., and the Cambrian explosion is considered by many to start with the first trilobites, about 530 Mya.) Testate amoebae are known from about 750 Mya (Porter and Knoll 2000). There are tracelike fossils more than 1,200 Mya in the Stirling Range Formation of Australia (Rasmussen et al. 2002). Eukaryotes (which have relatively complex cells) may have arisen 2,700 Mya, according to fossil chemical evidence (Brocks et al. 1999). Stromatolites show evidence of microbial life 3,430 Mya (Allwood et al. 2006). Fossil microorganisms may have been found from 3,465 Mya (Schopf 1993). There is isotopic evidence of sulfur-reducing bacteria from 3,470 Mya (Shen et al. 2001) and possible evidence of microbial etching of volcanic glass from 3,480 Mya (Furnes et al. 2004).

None of this is news to us. As the article correctly notes, there is evidence of multicellular life in the Doushantuo formation in China, dating to a few tens of millions of years prior to the Cambrian. 

Vernanimalcula guizhouena, for example, which is the earliest known Bilateria, is found in deposits dating from 600 to 580 million years ago. The Doushantuo formation also contains fossilized microscopic sponges and corals and various other aquatic microscopic fauna.

Far from resolving Darwin's dilemma, however, the discovery of these small and fragile organisms has only accentuated the problem, for it substantially undermines the common response that the Precambrian organisms were too small and too soft-bodied to be preserved. The same is true with respect to the testate (shelled) amoebae fossils that date back to the Cryogenian period (850-635 million years ago). None of the organisms mentioned by TalkOrigins represent transitional precursors to the forms that appear so abruptly during the Cambrian explosion.

In any case, as discussed in some detail here, the Ediacaran fauna are not broadly thought to be the ancestors of modern taxonomic groups such as the various metazoan phyla which appear explosively in the Cambrian radiation. The presence of these organisms, therefore, should offer no comfort to Darwinists. As Peter Ward has observed in On Methuselah's Trail: Living Fossils and the Great Extinctions,

[L]ater study cast doubt on the affinity between these ancient remains preserved in sandstones and living creatures of today; the great German paleontologist A. Seilacher, of Tübingen University, has even gone so far as to suggest that the Ediacaran fauna has no relationship whatsoever with any currently living creatures. In this view, the Ediacaran fauna was completely annihilated before the start of the Cambrian fauna." (p. 36)

Response #2: Transitional fossils exist.

The article states,

There are transitional fossils within the Cambrian explosion fossils. For example, there are lobopods (basically worms with legs) which are intermediate between arthropods and worms (Conway Morris 1998).

The famous Hallucigenia (pictured left) was a genus of the lobopods. They also included other well-known groups such as Aysheaia.

They certainly did have features in common with worms, including their long segmented bodies. At the same time, they had many features in common with arthropods. For example, a study of Miraluolishania haikouensis revealed that their eyes are very similar to those of arthropods (Liu et al., 2004). What we do not find, however, are numerous connecting forms leading from annelid worms to arthropod crustaceans (the arthropod subphylum likely to be the closest related to annelid worms due to their holoblastic form of cleavage). Indeed, lobopods appear in the Cambrian explosion at the same time as the arthropods.

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