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| Australian lungfish, Neoceratodus forsteri |
Once again, science has managed to comprehensively refute creationism and confirm evolution without even trying!
This time, they showed how the tetrapod limb evolved out of the fin of an ancestor of the Australian lungfish, Neoceratodus forsteri.
We know from fossils such as Tiktaalik that tetrapods first emerged from water as air-breathing, lobe-finned fish, but the evidence that the archetypal terrestrial vertebrate limb, with five digits, evolved from the fin of a lobe-finned fish was difficult to find. Now, an international team of researchers from the University of Konstanz (Germany), Macquarie University in Sydney (Australia) and the Stazione Zoologica Anton Dohrn in Naples (Italy) have shown that the embryo of the Australian lungfish, a descendant of the Sarcopterygian group of lobe-finned fish that closely resembles the fish that were around when the transition from aquatic to terrestrial vertebrates occurred, already has a 'hand' that goes on to develop into a fin.
The team published their findings in Science Advances a couple of days ago:
Abstract
How the hand and digits originated from fish fins during the Devonian fin-to-limb transition remains unsolved. Controversy in this conundrum stems from the scarcity of ontogenetic data from extant lobe-finned fishes. We report the patterning of an autopod-like domain by hoxa13 during fin development of the Australian lungfish, the most closely related extant fish relative of tetrapods. Differences from tetrapod limbs include the absence of digit-specific expansion of hoxd13 and hand2 and distal limitation of alx4 and pax9, which potentially evolved through an enhanced response to shh signaling in limbs. These developmental patterns indicate that the digit program originated in postaxial fin radials and later expanded anteriorly inside of a preexisting autopod-like domain during the evolution of limbs. Our findings provide a genetic framework for the transition of fins into limbs that supports the significance of classical models proposing a bending of the tetrapod metapterygial axis.
Woltering, Joost M.;Irisarri, Iker;Ericsson, Rolf;Joss, Jean M. P.;Sordino, Paolo;Meyer, Axel
Sarcopterygian fin ontogeny elucidates the origin of hands with digits
Science Advances; 6(34), EABC3510; 10.1126/sciadv.abc3510
Copyright: © 2020 The authors, Published by The American Association for the Advancement of Science.
Open access.
Reprinted under a Creative Commons Attribution 4.0 International licence (CC BY 4.0)
The news release from the University of Konstanz explains further:
The evolution of limbs with functional digits from fish fins happened approximately 400 million years ago in the Devonian. This morphological transition allowed vertebrates to leave the water to conquer land and gave rise to all four-legged animals or tetrapods – the evolutionary lineage that includes all amphibians, reptiles, birds and mammals (including humans). Since the nineteenth century several theories based on both fossils and embryos have been put forward trying to explain how this transformation unfolded. Yet, exactly how hands with digits originated from fish fins remained unknown.
[..]
Insights from embryonic development: limb “architect” genes
To solve the puzzle of how limbs emerged from fins during evolution researchers have focused on embryonic development. “During embryogenesis, a suite of ‘architect’ genes shapes an amorphous group of precursor cells into fully grown limbs”, explains Dr. Joost Woltering, first author on the study and an assistant professor in the Evolutionary Biology group at the University of Konstanz led by Professor Axel Meyer. The very same “architect” genes also drive fin development. However, because evolutionary changes have occurred in the activity of these genes, the developmental process produces fins in fish and limbs in tetrapods.
To compare this process in fins and limbs, the team studied such “architect” genes in the embryos of the Australian lungfish. “Amazingly, what we discovered is that the gene specifying the hand in limbs (hoxa13) is activated in a similar skeletal region in lungfish fins”, explains Woltering. Importantly, this domain has never been observed in the fins of other fish that are more distantly related to tetrapods. “This finding clearly indicates that a primitive hand was already present in the ancestors of land animals”.
Developmental patterns: differences and similarities
The lungfish “hand”, in spite of this modern genetic signature, only partially resembles the anatomy of tetrapod hands because it lacks fingers or toes. To understand the genetic basis for this difference the team went on to analyse additional genes known to be associated with the formation of digits, finding that one gene important for the formation of fingers and toes (hoxd13 – a “sister gene” to the above-mentioned hoxa13) appeared to be switched on differently in fins.
During tetrapod limb development, the hoxd13 gene is switched on in a dynamic manner. It first becomes activated in the developing pinky finger and then expands all the way throughout the future hand towards the thumb. This process coordinates the correct formation of all five fingers. While Joost Wolteringʼs team observed a similar activation pattern of this gene in lungfish fins, it did not show this expansion but only remained activated in exactly one half of the fin. Additional differences were found for genes that are normally switched off in digits. In lungfish fins these genes remain active, but on the opposite side of the domain where hoxd13 is activated.
It seems then that we owe our fingers and toes to these ancient ancestors of all terrestrial tetrapods. Something evolved to produce a limb capable of hauling a lungfish out of the water, maybe to escape predators or, more likely, to catch the crawling arthropods that had colonised the land first, provided the basis for the limbs of all amphibians, reptiles, mammals and birds and set the fundamental pattern of all tetrapod skeletons.
Evolution in action and no magic or deity required. A refutation of creationism again provided by science, incidentally and without intent or effort, simply by revealing the facts. Tweet
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