Fossil Said to Enlighten Evolution of the Ear
Did mammal ear bones evolve? If so, it was not a straightforward Darwinian progression. Authors of a paper in Science who announced a new Cretaceous mammal fossil from China had to invoke convoluted explanations to keep the evolution story intact.
Science Daily shows an artist’s conception of Maotherium, a chipmunk-sized mammal said to have lived 123 million years ago. The original paper by Ji et al claimed that this fossil “sheds light on the evolution of the definitive mammalian middle ear (DMME).”1 The reader can judge whether a convoluted story of evolution, loss and re-evolution constitutes enlightenment. Their chart shows a phylogenetic tree of mammals. Some of the groups have middle ear bones detached from the mandible (the DMME), while others have it attached via Meckel’s cartilage – a slender strip of cartilage that is dissolved in true mammals during embryonic development. Maotherium and a couple of other oddballs have a partially-ossified Meckel’s cartilage. These are mixed up in the timeline. The platypus lacks the cartilage, for instance, but subsequent mammal groups retain it. There doesn’t seem to be any rhyme or reason for the back-and-forth differences in the fossils. We do see occasional modern mice and humans that show a partially ossified cartilage that failed to dissolve away during development.
To explain the data from this and other mammal fossils in evolutionary terms, the authors had to invoke embryonic development, convergent evolution and regulatory network mutations. They offered a choice: either (1) the common ancestor of mammals already had the DMME and subsequent members lost it, or (2) the common ancestor lacked the DMME and up to three subsequent groups evolved it independently. They preferred option 1, but the scientists are not sure what this means. “This can be a new evolutionary feature,” suggested an article in PhysOrg. “Or, it can be interpreted as having a ‘secondarily [sic] reversal to the ancestral condition,’ meaning that the adaptation is the caused [sic] by changes in development.” Sounds like opposite interpretations are possible: evolution up, or evolution down.
Thomas Martin and Irina Ruf (U of Bonn) commented on this paper in the same issue of Science.2 Surprisingly, they began with Haeckel’s old biogenetic law (recapitulation theory) – and even referenced his 1866 book:
During their development, embryos of many species repeat evolutionary stages of their ancestors (1) [this is the Haeckel reference]. For example, in human embryos gill pouches are formed during early developmental stages. Developmental heterochrony—that is, the differing timing of developmental processes during embryonic growth—can lead to a premature fixation of ancestral character states and the retention of embryonic patterns in the adult. This process is believed to be an important driving force for evolution (2, 3). One of the key innovations in mammalian history is the evolution of the mammalian ear, leading to the most efficient hearing among vertebrates. On page 278 of this issue, Ji et al. (4) use an analysis of the Early Cretaceous mammal Maotherium to show how heterochrony routed the evolution of the definitive mammalian middle ear (DMME).
This paragraph reveals several things. First, Haeckel’s discredited biogenetic law, like a vestigial organ, still appears in the body of evolutionary literature. Second, the discredited belief that human embryos have gill slits is still around. Third, the idea that “fixation of ancestral character states” could paradoxically be called “a driving force for evolution” was not explained; presumably fixation is the opposite of force. And fourth, calling the mammalian ear a “key innovation” assumes evolution as a cause without demonstrating or explaining it. This line of thinking “sheds light on evolution” according to Ji et al. Martin and Ruf took the enlightenment to even higher levels in their last paragraph:
The approach of Ji et al. exemplifies recent studies that have combined paleontology and developmental biology to gain deep insight into evolutionary processes. These studies have shown that mammalian evolution was much more complex than had been thought a few years ago. Developmental processes played a central role in evolutionary changes in mammals, as recently shown for patterns of rodent teeth. The middle ear and mandible of Maotherium demonstrate that besides orderly evolution from primitive to derived characters, reversals to more primitive conditions are also to be expected. In the case of the DMME, the labile phase with multiple reversals appears to have ended with the evolution of the coiled cochlea in the inner ear of more derived ancestors of therian mammals (marsupials and placentals).
There’s a good dictionary word: labile. It means, simply, “changeable.” Simplifying this paragraph, the two profs have told us that looking into bones and embryos brought “deep insight.” The key insights are (1) evolution is a lot more complex than thought, and (2) evolution goes forward, backward and sideways (12/19/2007) – not (as Darwin thought) from primitive to advanced. We also learn from this paragraph not to look for evolution in genetic mutations, but in embryonic development and regulatory networks. There, we are told, happenstance can freeze ancestral states in the adult (assuming the embryo was retracing its evolutionary history, as Haeckel taught). We were told that multiple reversals can occur while evolution is in its “labile” (changeable) phase. Somehow, the unexplained appearance of a “coiled cochlea,” one of the most complex and remarkable organs in the mammalian body, put an end to the lability.
Did the popular reports pick up on these explanatory novelties that differ from classical Darwinian stories? Not a bit. The Science Daily article continued the enlightenment theme: “This new remarkably well preserved fossil … offers an important insight into how the mammalian middle ear evolved.” The study “sheds light on how complex structures can arise in evolution because of changes in developmental pathways.” The article briefly described the amazing clarity and efficiency of the mammalian ear, but then quickly returned to evolution: “To evolutionary biologists, an understanding of how the sophisticated and highly sensitive mammalian ear evolved may illuminate how a new and complex structure transforms through evolution.” How that new and complex structure “arose” was not explained. One can have faith in evolution, however, by looking at the bones through an imaginary time tunnel. “The unusual middle ear structure … is actually the manifestation of developmental gene mutations in the deep times of Mesozoic mammal evolution.” Deep times or not, a mutation is a chance occurrence without cause or foresight. These scientists, therefore, just explained “middle ear structure” by chance – i.e., stuff happens.
No matter the convoluted back-and-forth route it took, and the belief in the chance emergence of functional complexity, evolution survived in these articles as source of enlightenment. PhysOrg in its write-up repeated the line that this fossil “sheds light” on evolution. It “illuminates” evolution. It “elucidates” evolution. And with illumination, they believe, comes understanding.
1. Ji, Luo, Zhang, Yuan and Xu, “Evolutionary Development of the Middle Ear in Mesozoic Therian Mammals,” Science, 9 October 2009: Vol. 326. no. 5950, pp. 278-281, DOI: 10.1126/science.1178501.
2. Martin and Ruf, “Paleontology: On the Mammalian Ear,” Science, 9 October 2009: Vol. 326. no. 5950, pp. 243-244, DOI: 10.1126/science.1181131.
Today’s gnostics and diviners are the evolutionary biologists. They use fossils and embryos as divination tools to gain secret knowledge and enlightenment. They know their gill slits and ancestral cartilages better than any Babylonian knew the folds in a cow liver. They bluff their superiority by showing they can toss around heavy words from the inner sanctum like heterochrony, homoplasy, labile, and impenetrable jargon thickets (to the layman) like “The trechnotherian clade of living therians and spalacotheroids is one of 20 or so Mesozoic mammaliaform clades.” Once the thicket is cleared, though, one finds a group of fallible humans believing a mystery religion full of miracles. The exquisite middle ear bones arose. The coiled cochlea emerged. The highly efficient system of hearing evolved. Maybe it didn’t evolve from simple to complex along a straightforward route like Darwin thought, but it evolved. Stuff happens! Complex systems emerge from the darkness! “It evolved” is the password to illumination, to membership in the Darwin Illuminati.*
*Pronunciation: ill, loo, mean, naughty.