New Precambrian Embryos Are Equivocal at Best
Precambrian embryos? Old news. Paleontologists have been looking at them for over a decade, yet the Cambrian explosion remains one of the strongest empirical challenges against Darwin's theory. A new paper tries to show that some of them were "possibly" embryos of metazoan animals that emerged long before the explosion, thereby lengthening the time during which evolution could have worked its magic.
In Darwin's Doubt, Stephen Meyer pointed to the embryos as evidence against the "artifact hypothesis." This hypothesis tried to explain the lack of Precambrian ancestors as an artifact of poor fossil preservation. "If these strata could preserve embryos," Meyer said, "then they should have preserved fully developed animals -- at least, if such animals were present at the time" (p. 68). Illustra Media's film Darwin's Dilemma includes Paul Chien, who worked with Dr. J.Y. Chen at the discovery site in China, dissecting some of the fossils, showing them under the microscope to be characteristic of sponge embryos.
The embryos come from the Precambrian Doushantuo formation in China, dated at 600 million years old -- about 60 million years before the onset of the Cambrian Explosion. This was the Ediacaran period, populated by strange-looking sessile colonies of unknown organisms. Meyer discusses the Ediacaran in detail in his book, documenting that most leading evolutionary paleontologists consider the Ediacaran creatures to be unrelated to the Cambrian animals. Meyer readily acknowledges the presence of sponges and possibly two other phyla before the explosion. The problem for evolutionists is to explain the geologically sudden appearance of almost twenty new phyla with new body plans more complex than sponges or anything else that came before - animals with jointed legs, guts, eyes, hard parts, and locomotion. Some of the phyla, like trilobites and Marrella, exhibit bilateral symmetry. These were the first bilaterians: a group that includes us.
Darwin himself confessed that "if numerous species, belonging to the same genera or families, have really started into life all at once, the fact would be fatal to the theory of descent with slow modification through natural selection" (quoted in Meyer, p. 17). His disciples ever since have scoured the earth for evidence of the missing Cambrian ancestors in the strata below. All they have found are microbes, sponges, the Ediacaran biota, and the embryos. A few small shelly fossils appear at the base of the Cambrian (see Debating Darwin's Doubt, Chapters 13-14, for details), but experts consider the Precambrian evidence to be insufficient to account for the profusion of new body plans in the Cambrian explosion.
To muffle the explosion, evolutionists have used two strategies to construct a "long fuse" of gradual evolutionary experimentation. One has been the molecular clock. Molecular estimates of mutation rates, they say, show that the ancestors must have existed, even without fossil evidence. The argument is circular; it relies on evolution to try to prove evolution. Even so, last October, we discussed a Current Biology paper by Telford, Donoghue, and Yang who showed after a detailed analysis that molecular clock data is too imprecise to draw any conclusions.
The other strategy has been to conjure complex animals out of the Precambrian fossil record. In Darwin's Doubt, pp. 90-92, Meyer showed how David Bottjer of the University of Southern California was roundly criticized by his colleagues in 2005 for making too much of an enigmatic fossil named Vernanimalcula. Bottjer had labeled it an early bilaterian, but most others considered it to be irrelevant to the Cambrian explosion (see Casey Luskin's article from 2012 that says Vernanimalcula is possibly not even a fossil at all). Last year we showed Bottjer still suggesting Vernanimalcula was a bilaterian.
Now, in a paper in Geology co-authored by three colleagues from China and an imaging expert in France, Bottjer has more show and tell from Doushantuo. Strangely, he omits mention of Vernanimalcula, noting that for the alleged Precambrian bilaterians, "most of them are not yet widely accepted." (The reference for that comment is to a paper with a very harsh title, "A merciful death for the 'earliest bilaterian,' Vernanimalcula." Clearly the authors, Bengtson, Cunningham, Yin, and Donoghue, disagreed that it was ever "widely accepted.")
The Geology paper presents three new embryos that the authors claim show a cleavage pattern characteristic of metazoan animals, perhaps even bilaterians. Phys.org shares the gist of the new claim:
In their article for Geology, Zongjun Yin and colleagues report new Doushantuo embryo-like fossils. They used high-resolution synchrotron radiation X-ray microtomography to reconstruct three-dimensional structures of the fossils, and the results demonstrate that these fossils preserve unique features directly comparable to living animal embryos that utilize a special kind of cell division pattern known as discoidal cleavage. Given that discoidal cleavage only occurs in animal embryos, the biological affinities of these fossils are probably animals. [Emphasis added.]
Since these embryos are in the Doushantuo formation (dated 600 million years old), there must have been animals alive back then, if not earlier. Problem solved? Well, look at the images and think about the interpretation. First some terminology. In their pre-gastrula stages, embryos show either holoblastic cleavage (where the first cells all look the same size) or meroblastic cleavage (where one cell becomes a large "yolk" for the others). Discoidal cleavage is a type of meroblastic cleavage where the cells sit on top of the yolk cell without penetrating it. Identifying the type of cleavage in these phophatized embryos is the key to interpreting them, they say:
Recently, the debate on the EDEFs [Ediacaran Doushantuo Embryo-like Fossils] has begun to crystallize into two competing interpretations: (1) that the EDEFs represent crown metazoans, or (2) that they represent stem metazoans or nonmetazoan holozoans. These two competing interpretations have very different implications for the timing and tempo of animal diversification [i.e., the Cambrian explosion]. If the EDEFs are crown metazoans, a deep Precambrian history of animals is implied, whereas if they are stem or non-metazoans, the fossils do not reduce the gap between molecular clock estimates and the fossil evidence for the early divergence of metazoans. It is difficult to reconcile these competing interpretations because these morphologically simple EDEFs yield very little phylogenetic information.
And so the authors argue that the appearance of discoidal cleavage suggests that the embryos could be crown metazoans, implying "a deep Precambrian history of animals." Here are some issues:
Only one of the 3 microfossils shows "possible" discoidal cleavage; the others are ambiguous, judging from the images; they only show some cells smaller than one larger cell.
The prime example has 12 smaller cells appearing in an indentation in the large cell. They cannot rule out, however, "taphonomic bias" (artifacts from fossilization) or unequal cleavage in a holoblastic embryo.
Cleavage patterns vary widely among multicellular organisms. While no instances of discoidal cleavage have been "reported" outside of bilaterians, they are not necessarily diagnostic of bilaterians.
Many bilaterians undergo holoblastic cleavage: annelids, mollusks, echinoderms, tunicates, amphibians, some fish, and placental mammals. Some of these are Cambrian phyla; others are advanced vertebrates.
Holoblastic cleavage organisms can be organized into 5 kinds within two categories (see Wikipedia); in other words, there are variations that might be interpreted as meroblastic or discoidal in fossil conditions.
Discoidal cleavage is not universal among bilaterians; it appears in widely separated groups including some fish, sharks, birds, reptiles and monotremes (like platypus).
Fossil embryos represent only a snapshot in time of cells turned to stone. The scientists cannot watch the developmental process unfold.
The authors are hanging an awful lot of interpretation, therefore, on one 250-micrometer fossil that they acid-washed and scanned with radiation, then processed for visualization in software. If this little piece of rock is so important, why are there not hundreds of them in all stages of development? Most importantly, where are the adults? Clearly, the Doushantuo formation was capable of fossilizing higher life stages in great detail. Their absence seems better evidence than the presence of a questionable embryo that, until proven otherwise, could be another sponge. Notice the subjective interpretation in the paper, amounting to little more than possibility thinking:
Our findings support the conclusion that at least some EDEFs possibly represent crown-animals, although their phylogenetic affinity cannot be established because discoidal-type meroblastic cleavage has evolved independently in a variety of animal groups, e.g., scorpions and cephalopods as well as many vertebrates, including some fishes and amniotes.
What is it, therefore, about discoidal cleavage that is worth getting excited about? Keep in mind that Bottjer has demonstrated a strong antipathy to intelligent design, dismissing Darwin's Dilemma as "a creationist movie" in spite of its fully scientific arguments; he took part in the effort to block its showing at the California Science Center; and he promoted Vernanimalcula beyond the evidence enough to rouse the censure of his colleagues.
These points aside, what's convincing is the positive evidence for design in animal development, complex body plans and a biosphere that interacts at all levels on a privileged planet in a finely tuned universe. That's huge. That's Undeniable.
Image credit: Dr. Zongjun Yin, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences via Phys.org.