Darwin Fish Lacks Tetrapod Legs
You can see a transition between a fish and a land creature in fossils and genes only if you have a vivid imagination.
Evolutionists made a big media push this month to promote their narrative of a Great Transformation between fish and land animals. One came in a new science paper on genetic evidence co-authored by a champion of that narrative, Dr. Neil (“Your Inner Fish”) Shubin. Another came from fossil evidence about a star witness, the extinct “stem tetrapod” Acanthostega, which comes after Tiktaalik in the narrative. As we shall see, neither evidence serves as a strong prop for the stage play of tetrapod origins.
In Nature, Aditya Saxena and Kimberly L. Cooper announce that the fin-to-limb transition is “within our grasp.” Why? A new paper by Nakamura, Shubin et al., also published in Nature, shows that Hox genes between zebrafish and mice “have more in common than was previously thought.” The particular developmental genes relate to fin rays in the fish and finger digits in the mammal. Saxena and Cooper acknowledge, though, that the resulting structures comprise different types of bone (endochondral bone in the fish, dermal bone in the mouse). After looking at Shubin’s evidence, they backtrack from the headline, admitting that the evidence is not really “within our grasp.”
Some caution should be taken in the interpretation of these data. Because zebrafish are highly derived compared with more-basal fishes, it is possible that the role of hox13 transcription factors in the development of fin rays is a recent zebrafish acquisition. It will be important, where possible, to perform some of the same fate-mapping and gene loss-of-function experiments in fish species, such as the paddlefish and gar, that diverged closer to the shared ancestor with tetrapods and that have fin skeletons with more similarities to ancestral tetrapods. Fortunately, these exciting questions are emerging just as CRISPR–Cas9 genome-editing technologies are becoming options for a variety of unusual model species. The answers may soon be within our grasp.
If the transition were solved, Nakamura and Shubin would not have begun their paper with these confessions:
Understanding the evolutionary transformation of fish fins into tetrapod limbs is a fundamental problem in biology. The search for antecedents of tetrapod digits in fish has remained controversial because the distal skeletons of limbs and fins differ structurally, developmentally, and histologically. Moreover, comparisons of fins with limbs have been limited by a relative paucity of data on the cellular and molecular processes underlying the development of the fin skeleton.
The paper reveals only the barest of possible connections in certain developmental genes. Employing the power of suggestion, they say: “These discoveries reveal a cellular and genetic connection between the fin rays of fish and the digits of tetrapods and suggest that digits originated via the transition of distal cellular fates.” Assuming a transition, however, is not the same as providing evidence for it. “Neil Shubin and colleagues have compared Hox function in fins and limbs by performing cell lineage and knockout assays of Hox genes in zebrafish fins,” the Editor’s Summary states. “They find that zebrafish deficient in HoxA and HoxD gene clusters show reduced fin rays and increased endochondral elements.” That’s negative evidence in a “derived” fish. Mice, in turn, are mammals far removed from the presumptive first tetrapod – hardly evidence for creative inventions of shoulders, wrists, and digits. It’s not surprising that common upstream switches would have detrimental effects on downstream products.
All fossils of Acanthostega come from eastern Greenland, where Jennifer Clack found them in 1987. Till now, paleontologists assumed they were adult skeletons. Wrong; a new find “rewrites the tetrapod move to land,” Science Daily says. A new analysis of the limited number of available fossils in X-rays by researchers at Uppsala University suggests that the individuals were juveniles.
The tetrapods are four-limbed vertebrates, which are today represented by amphibians, reptiles, birds and mammals. Early tetrapods of the Devonian period (419-359 million years ago) are of great interest to palaeontologists: they were the earliest vertebrate animals that ventured onto land, paving the way for all future vertebrate life on land. The move from water to land must have affected every aspect of the biology of these animals, but until now there has been no serious attempt to investigate their life histories — how long they lived or whether they had an aquatic juvenile stage, for example. Well-preserved skeletons are rare and it has simply been assumed that they represent adults.
What is the impact of undermining that assumption? For one, it now becomes unlikely that the juveniles could have ventured onto land. And since we now realize we don’t have adult specimens of Acanthostega, Live Science notes, “it’s impossible to say whether they were aquatic or terrestrial animals” at all. Sophie Sanchez at Uppsala connects the dots: “This means that we need to find the adult fossils before being able to build up theories on the tetrapod move to land,” she said. It’s like being back to square one.
None of the articles or papers addressed the problem of tetrapod trackways found in Poland that predate Tiktaalik by 20 million Darwin years in the evolutionary timeline (see Casey Luskin’s article on that in Evolution News & Views). Nor did they explain whether living fish like mudskippers (see YouTube clip from National Geographic) are going through a great transformation now (see Luskin’s response to fish-amphibian similarities in ENV, 2011). A Canadian submersible found a bright red deep-sea fish walking on the seafloor (YouTube), apparently oblivious to ambitions of invading the land.
The tetrapod transition narrative remains an “evolutionary gem” for Darwinians (see Casey Luskins listing from ENV, 2010). But with wrong assumptions and only flimsy suggestions that fish learned to walk by mutation and natural selection, the story seems overhyped (see Casey Luskin’s critique of the Hox gene evidence from ENV, 2014). David Klinghoffer reported an “everything-you-know-is-wrong” discovery about early tetrapod backbones in 2013 on ENV. How many other hits could this already-challenged narrative survive in the future?
Update 9/14/16: A week after Shubin’s paper, another paper by Jennifer Clack and Per Ahlberg appeared in Nature (Sanchez et al.) reinforcing the juvenile age of the Acanthostega fossils. The editor’s summary says that the individuals, six years old at the oldest, were “obligately aquatic”, In a companion piece in Nature, Nadia B. Fröbisch says that the mass kill of the fish occurred in a “catastrophic flood event”. Neither of the articles provides any specific evidence for evolution beyond speculation in future tense, such as, “Their results will provide a deeper understanding of the development and evolution of our four-legged forerunners.”
Like the YouTube clips show, it’s not unusual to see some fish using strong fins for locomotion on land. The only ones Darwinians get excited about are the ones they can fit into their timeline at the presumed spot where the transition occurred. They used to think coelacanth was the innovator of precocious limbs, until living ones were seen using their bony fins for swimming, not walking (4/18/13, 1/25/14).
Creationists see the world populated with an enormous variety of creatures, with some potential for variability within their kinds, each well adapted to its habitat. Only Darwin’s ideology tries to connect them in a branching ancestral sequence. Instead of looking for tiny clues here and there that offer little more than suggestions, biologists should examine the big picture: how could blind, unguided processes produce unified organisms with senses, locomotion, digestion, and reproduction? Douglas Axe calls such entities (whether animals or robots) “busy wholes” and “whole projects” in chapter 6 of his new book Undeniable, a good read about functional coherence that we all know by intuition comes from a personal, intelligent source. Axe provides abundant evidence from his own research on proteins to show why our intuition is scientifically correct.