The "Ghost of Teleology": Molecular Data Wreak Havoc on the Tree of Life
A new article in the science magazine Nautilus, "Evolution, You're Drunk: DNA studies topple the ladder of complexity," reminds readers of the false yet common assumption that evolution always progresses towards greater complexity. This is a basic point that virtually every undergraduate learns in studying evolution. However, the vehicle that this article uses to make the point is intriguing. Writer Amy Maxmen shows how convoluted animal phylogenetic trees, seemingly required by the genetic data, suggest that important and highly complex biological structures have been gained and lost many times in evolutionary history. From a Darwinian perspective that is perplexing:
Then molecular analyses did something else. They rearranged the order of branches on evolutionary trees. Biologists pushed aside trees based on how similar organisms looked to one another, and made new ones based on similarities in DNA and protein sequences. The results suggested that complex body parts evolved multiple times and had also been lost. One study found that winged stick insects evolved from wingless stick insects who had winged ancestors. Another analysis suggested that extremely simple animals called acoel worms -- a quarter inch long and with just one hole for eating and excreting -- evolved from an ancestor with a separate mouth and anus. Biologists' arrow of time swung forward and backward and forward again.Stephen Meyer uses exactly the same example in Darwin's Doubt, noting that the animal tree suggested by the presence or absence of a coelom (a major metazoan body cavity) conflicts sharply not just with the genetic data, but with another tree based upon whether an animal molts:
Douglas Theobald claims in his "29+ Evidences for Macroevolution" that "well-determined phylogenetic trees inferred from the independent evidence of morphology and molecular sequences match with an extremely high degree of statistical significance."But the article in Nautilus explains that quirky phylogenetic data is hardly limited to the presence or absence of a coelom, or the origin of winged insects. In fact, some basic animal traits -- the brain and the nervous system -- aren't distributed among animals in a treelike pattern:
In reality, however, the technical literature tells a different story. Studies of molecular homologies oft en fail to confirm evolutionary trees depicting the history of the animal phyla derived from studies of comparative anatomy. Instead, during the 1990s, early into the revolution in molecular genetics, many studies began to show that phylogenetic trees derived from anatomy and those derived from molecules oft en contradicted each other.
Probably the most protracted conflict of this type concerns a widely accepted phylogeny for the bilaterian animals. This classification scheme was originally the work of the influential American zoologist Libbie Hyman. Hyman's view, generally known as the "Coelomata" hypothesis, was based on her analysis of anatomical characteristics, mainly germ (or primary tissue) layers, planes of body symmetry, and especially the presence or absence of a central body cavity called the "coelom," which gives the hypothesis its name. In the Coelomata hypothesis, the bilaterian animals were classified in three groups, the Acoelomata, the Pseudocoelomata, and the Coelomata, each encompassing several different bilaterian animal phyla.
Then, in the mid 1990s, a very different arrangement of these animal groups was proposed based on the analysis of a molecule present in each (the 18S ribosomal RNA; see Fig. 6.1b). The team of researchers who proposed this arrangement published a groundbreaking paper in Nature with a title that surprised many morphologists: "Evidence for a Clade of Nematodes, Arthropods and Other Moulting Animals." The paper noted the conventional wisdom, based on Hyman's hypothesis, that arthropods and annelids were closely related because both phyla had segmented body plans. But their study of the 18S ribosomal RNA suggested a different grouping, one that placed arthropods close to nematodes within a group of animals that molt, which they called "Ecdysozoa." This relationship surprised anatomists, since arthropods and nematodes don't exactly look like kissing cousins. Arthropods (such as trilobites and insects) have coeloms, whereas nematodes (such as the tiny worm Caenorhabditis elegans) do not, leading many evolutionary biologists to believe nematodes were early branching animals only distantly related to arthropods. The Nature paper explained how unexpected this grouping of arthropods and nematodes was: "Considering the greatly differing morphologies, embryological features, and life histories of the molting animals, it was initially surprising that the ribosomal RNA tree should group them together." (pp. 122-123)
Late last year, the animal evolutionary tree quaked at its root. A team led by Joseph Ryan, an evolutionary biologist who splits his time between the National Genome Research Institute in Bethesda, Md. and the Sars International Center for Marine Molecular Biology in Bergen, Norway, analyzed the genome from a comb jelly, Mnemiopsis leidyi, a complex marine predator with muscles, nerves, a rudimentary brain, and bioluminescence, and found that the animals may have originated before simple sponges, which lack all of those features.Now Dr. Ryan's team rightly point out that this data contradicts the idea that animals "evolve to be increasingly complex." But few in evolutionary biology would hold neo-Darwinian evolution to that standard. Yes, "the idea that complex parts like a brain and nervous system -- including nerve cells, synapses, and neurotransmitter molecules -- could evolve separately multiple times perplexes evolutionary biologists." But the reason for that is NOT simply "because parts are gained one at a time" but in large part because it's very unlikely for highly complex parts to evolve separately multiple times, and it's also very strange to see animals losing such important complex parts. Moreover, when complex parts that are shared by different animals aren't distributed in a treelike pattern, that wreaks havoc on the assumption of homology that's used to build phylogenetic trees. In other words, this kind of extreme convergent evolution refutes the standard assumption that shared biological similarity (especially complex biological similarity like a brain and nervous system) implies inheritance from a common ancestor.
If comb jellies evolved before sponges, the sponges might have lost the complexity that the ancestor uniting them and comb jellies possessed. Or, that ancestor -- the ancestor of all living animals -- had the genes to build brains and muscles, but did not form those parts, and neither did sponges. If this is true, then comb jellies deployed the genome they inherited to build a brain, nervous system, and muscles, independent of other animals. There's some support for this possibility: A unique set of genes seems to underlie comb jellies' muscles.
Both hypotheses run counter to scenarios in which organisms evolve to be increasingly complex. In one, a complex nervous system and muscles were lost in the sponges. In the other, the sponges had the genetic capability for complex features but stayed simple, while a more primitive group, the comb jellies, acquired brains and muscles that help them chase down prey. Furthermore, the idea that complex parts like a brain and nervous system -- including nerve cells, synapses, and neurotransmitter molecules -- could evolve separately multiple times perplexes evolutionary biologists because parts are gained one at a time. The chance of the same progression happening twice in separate lineages seems unlikely -- or so biologists thought. "Traditional views are based on our dependence on our nervous system," says Ryan. "We think the nervous system is the greatest thing in the world so how could anything lose it," he says. "Or, it's the greatest thing in the world, so how could it happen twice."
If brains and nervous systems evolved multiple times, this undermines the main assumptions used in constructing phylogenetic trees, calling into question the very basis for inferring common ancestry. It also suggests what Simon Conway Morris called the "ghost of teleology" which is "looking over th[e] shoulders" of scientists who repeatedly find such convergence in nature:
During my time in the libraries I have been particularly struck by the adjectives that accompany descriptions of evolutionary convergence. Words like, 'remarkable', 'striking', 'extraordinary', or even 'astonishing' and 'uncanny' are commonplace...the frequency of adjectival surprise associated with descriptions of convergence suggests there is almost a feeling of unease in these similarities. Indeed, I strongly suspect that some of these biologists sense the ghost of teleology looking over their shoulders.When Darwinian theory tells us that crucial and complex features like brains or nervous systems evolved independently -- or almost as weirdly, evolved and were repeatedly lost throughout life's history -- maybe, it's time for the "ghost of teleology" to make an appearance in the form of common design.
(Simon Conway Morris, Life's Solution: Inevitable Humans in a Lonely Universe, pp. 127-128 (Cambridge Press, 2003).)