I recently read Carl Zimmer’s response to my critique of his November, 2006 article in National Geographic. In this post I will discuss Zimmer’s response to me regarding embryology and developmental biology. The embryonic hourglass is the idea that vertebrate embryos (like those of fish, amphibians, reptiles, birds, and mammals) start off developing very differently, converge with some similarities at the pharyngular stage, and then again diverge. I stated in my original article that “vertebrate embryos start off quite differently,” but that “Zimmer’s diagram selectively displays embryos from the encircled stage where they are most similar.” The implication is that this falsifies the idea that evolution proceeds by tacking on new stages of development because these vertebrate groups start off with different forms of development from their very beginning.
Zimmer justifies his exclusion of the differences between earlier stages where embryos are different by arguing that the embryonic hourglass is an imaginary idea, stating: “As fellow scienceblogger PZ Myers has clearly explained, the differences in the earliest stages are superficial.” But are the early embryonic differences truly just “superficial”? As Richardson et al. explain:
According to recent models, not only is the putative conserved stage followed by divergence, but it is preceded by variation at earlier stages, including gastrulation and neurulation. This is seen for example in squamata, where variations in patterns of gastrulation and neurulation may be followed by a rather similar somite stage (Hubert 1985). Thus the relationship between evolution and development has come to be modelled as an “evolutionary hourglass” (Fig. 1; Elinson 1987; Duboule 1994; Collins 1995).
Compare how similar Richardson’s diagram of the embryonic hourglass is to the diagram given in Jonathan Wells’ book Icons of Evolution:
Table: Demonstration that Other Developmental Biologists Have Similarly Promoted the Embryonic Hourglass, where Vertebrate Embryos Start Off Very Differently:
|The Embryonic Hourglass According to Richardson et al., “There is no highly conserved embryonic stage in the vertebrates: implications for current theories of evolution and development,” Anatomy and Embryology, (1997) 196:91–106.||The Embryonic Hourglass According to Jonathan Wells, Icons of Evolution: Why Much of What We Teach about Evolution is Wrong (Regnery, 2000).|
Clearly I am not inventing the evolutionary hourglass, despite any suggestions that Zimmer might make in that direction. Yet Zimmer tries to minimize the differences between early vertebrates during development, stating:
In many cases, the difference is simply whether an embryos develops as a mass of cells, or a mass of cells sitting atop a yolk. Whatever the differences in how the earliest embryos look, they undergo the same core steps of development, known as gastrulation. And the same genes control that process. In other words, what we see in the earliest stages of vertebrate embryos today are variations on an ancestral theme.
So the only lines of evidence Zimmer cites in favor of early vertebrate similarities include (1) the fact that they all undergo gastrulation, and (2) the same genes control the process. Claim (2) is quite odd since one of my developmental biologist friends tells me that we only vaguely understand the genetic processes that direct gastrulation in the earliest pre-pharyngular stages of developmental in verbebrates, so Zimmer must think he knows things here that most developmental biologists don’t know (or he’s simply mistaken or he’s bluffing).
Zimmer then claims that such genetic similarity implies “variations on an ancestral theme” or “the standard evolutionary process.” That’s one way to view it, or perhaps the “variations on a theme” implies exactly what we might expect from a creative designer. Yet another pro-ID embryologist friend sent me the following regarding differences in vertebrate embryos prior to the pharyngular stage:
For one thing the cleavage patterns are very different. For instance, both amphibians and mammals have holoblastic cleavage (where the entire egg mass is divided up) but cleavage) but mammals division pattern is rotational where the amphibian is radial (pictures help here) Also, mammals separate cells to become the amnion, chorion and blastocyst whereas amphibians don’t. Fish, reptiles and birds alternatively have meroblastic or incomplete cleavage (where a large portion of the egg mass is left as a big yolk that the embryo develops on top of) but there are differences here too.
But what about gastrulation? Jonathan Wells explains that “in 1987, Richard Elinson reported that frogs, chicks, and mice ‘are radically different in such fundamental properties as egg size, fertilization mechanisms, cleavage patterns, and [gastrulation] movements.'” Additionally, in 2000 Andres Collazo wrote in Systematic Biology that there are many significant differences between the early stages of vertebrate development. He writes that “early development varies far more than has been previously thought” and cites the same landmark paper cited by Jonathan Wells to document these differences:
Recent workers have shown that early development can vary quite extensively (Raff et al., 1991), even within closely related species, such as sea urchins (Raff et al., 1991; Wray and McClay, 1989), amphibians (Collazo, 1990; Elinson, 1987), and vertebrates in general (Elinson, 1987). By early development, I refer to those stages from fertilization through neurolation (gastrulation for such taxa as sea urchins, which do not undergo neurulation). Elinson (1987) has shown how such early stages as initial cleavages and gastrula can vary quite extensively across vertebrates.
(Andres Collazo, “Developmental Variation, Homology, and the Pharyngula Stage,” Syst. Biol. 49(1):3-18, 2000, emphasis added.)
According to Collazo these early developmental differences “can vary quite extensively across vertebrates.” That sounds far from “superficial” differences, as Zimmer and PZ Myers reportedly put it. Zimmer’s blog response continued to obscure the differences between early stages of embryos. Zimmer and Myers are clearly puffing in order to diminish the differences between early vertebrate embryo stages.
Convergent Evolution and Evo-Devo
Finally, Zimmer tries to give a convincing account of evo-devo by noting that the same genes control growth of various limb-buds:
[M]any of the same genes are at work in fins, hands, and wings. The differences emerge thanks to the differences in when and where in the limb bud the genes produce their proteins. This is precisely the sort of evolution scientists are talking about when they refer to tinkering with genes that control development.
Truly, this is a fascinating hypothesis. But why must evolution be the explanation? Paul Nelson and Jonathan Wells observe that “[a]n intelligent cause may reuse or redeploy the same module in different systems.” Why couldn’t an intelligent cause re-use the same module to produce these different types of limbs?
Zimmer’s article focuses on vertebrate development. But the case for intelligent design in limb-bud controlling genes gets stronger when one realizes that the same regulatory genes are used to control limb growth in organisms far more diverse than vertebrates: mammals, urochordates, sea urchins, insects, annelid worms, and onycophorans, all use a similar regulatory gene to control limb growth. But they have radically different types of limbs, making this either a case of radically extreme “convergent evolution” or simple common design. (See Paul Nelson and Jonathan Wells, “Homology in Biology,” in Darwinism, Design, and Public Education (Michigan State University Press, 2003), pg. 316.) As plant geneticist from the Max Plank Institute, Wolf-Ekkehard Lönnig, wrote in Dynamical Genetics, “No theorist in evolutionary biology will ever derive chicken and insects from a winged common ancestor, and yet, clearly related sequences are specifically expressed in wing buds and imaginal disks.” Can Zimmer’s hypothesis account for such extreme convergence? Or perhaps again, Zimmer knows something most other biologists don’t know.