Another Gap Closes - The Evolution of Paired Limbs
No God(s) Needed
Where Do Our Limbs Come From?
Creationism, on the other hand, beyond childish hand-waving denialism, can't even offer an explanation for intermediate forms, since it claims all species were created as they are today, without ancestors, so there should be no evidence of progression through a gradual series of change, and an omniscient, omnipotent creator would not need to constantly revise and refine its creation, nor create species destined to become extinct and replaced by new ones.
That's just one reason, amongst many, why the archaeological and palaeontological record refutes the childish notion of special creation, without ancestry, by an omniscient creator making chemistry and physics do something they couldn't do without it.
So, when an international collaboration of scientists fills a gap in our knowledge, with new clues about the origin of paired appendages – a major evolutionary step that remains unresolved and highly debated, we can be confident it fits neatly into an evolutionary framework and completes a segment of the evolutionary picture of terrestrial tetrapods evolving from lobe-finned fish. It also, quite incidentally and without intent, refutes creationism yet again.
The scientists who have just produced this additional confirmation included scientists from the University of Colorado School of Medicine. Their findings have just been published, open access, in the journal Nature.
Their research and its significance are explained in a University of Colorado press release:
That question – where do our limbs come from? – has been subject of debate for more than 100 years. In 1878, German scientist Carl Gegenbaur proposed that paired fins derived from a source called the gill arch, which are bony loops present in fish to support their gills. Other scientists favor the lateral fin fold hypothesis, concluding that lateral fins on the top and bottom of the fish are the source of paired fins.This has become a topic that comes with bit of controversy, but it’s really a very fundamental question in evolutionary biology: Where do our limbs come from?
It is a highly active research topic because it’s been an intellectual challenge for such a long time. Many big labs have studied the various aspects of how our limbs develop and have evolved.
Professor Christian Mosimann, PhD, corresponding author
Associate professor and Johnson Chair in the Department of Pediatrics
Section of Developmental Biology
Anschutz Medical Campus, University of Colorado, Aurora, CO, USA
Among those labs are Dr. Mosimann’s colleagues and co-authors, Tom Carney, PhD, and his team at the Lee Kong Chian School of Medicine at Nanyang Technological University in Singapore.
Chasing the odd cells
For Mosimann, the inquiry into where limbs come from is an offshoot of other research conducted by his laboratory on the CU Anschutz Medical Campus. In his laboratory, his team uses zebrafish as a model to understand the development from cells to organs. He and his team study how cells decide their fate, looking for explanations for how development can go awry leading to congenital anomalies, in particular cardiovascular and connective tissue diseases.
Along the way, Mosimann and his lab team observed how a peculiar cell type with features of connective tissue cells, so-called fibroblasts that share a developmental origin with the cardiovascular system, migrated into specific developing fins of the zebrafish. It turns out that these cells may support a connection between the competing theories of paired appendage evolution.We always knew these cells were odd. There were these fibroblast-looking cells that went into the so-called ventral fin, the fin at the belly of the developing zebrafish. Similar fibroblast cells didn’t crawl into any other fin except the pectoral fin, which are the equivalent of our arms. So we kept noticing these peculiar fibroblasts, and we could never make sense of what these were for many years.
The embryo has features that are still ancient remnants that they have not lost yet, which provides insight into how animals have evolved. We can use the embryo to learn more about features that just persist today, allowing us to kind of travel back in time. We see that the body has a fundamental, inherent propensity to form bilateral, two-sided structures. Our study provides a molecular and genetic puzzle piece to resolve how we came to have limbs. It adds to this 100-plus year discussion, but now we have molecular insights.
There are labs on this on this paper that work on musculoskeletal diseases, toxicology, fibrosis. We work on cardiovascular, congenital anomalies, cardiopulmonary anomalies, limb development, all related to our interest on the lateral plate mesoderm. And then together, you get to make such fundamental discoveries. And that's where team science enables us to do something that is more than just the sum of the parts.
Professor Christian Mosimann.
The Mosimann lab has developed several techniques to track cell fates during development in pursuit of their main topic, which is an improved understanding of how the embryonic cell layer, called the lateral plate mesoderm, contributes to diverse organs. The lateral plate mesoderm is the developmental origin of the heart, blood vessels, kidneys, connective tissue, as well as major parts of limbs.
The paired fins that form the equivalent of our arms and legs are seeded by cells from the lateral plate mesoderm, while other fins are not. Understanding how these particular fins became more limb-like has been at the core of a long-standing debate.
Developing new theories
Hannah Moran, who is pursuing her PhD in the Cell Biology, Stem Cells and Development program in the Mosimann lab, adapted a method of tracking lateral plate mesoderm cells that contribute to heart development so that researchers could track the peculiar fibroblasts related to limb development.My primary research project focuses on the development of the heart rather than limb development, but there was a genetic technique that I had adapted to map early heart cells, and so we were able to implement that into mapping where the mysterious cells of the ventral fin came from. And turns out, they are also from the lateral plate mesoderm.
Hannah Moran, Co-author
Department of Pediatrics
Section of Developmental Biology, Anschutz Medical Campus, University of Colorado, Aurora, CO, USA
This crucial discovery provides a new puzzle piece to the big picture of how we evolved our arms and legs. Increasing evidence supports a hypothesis of paired appendage evolution called the dual origin theory.Our data fit nicely into this combined theory, but it can also stand on its own with the lateral fin theory. While paired appendages arise from the lateral plate mesoderm, that does not rule out an ancient connection to unpaired, lateral fins.
I wouldn’t say we’ve solved the question, or even disproven either existing theory. Rather, we’ve contributed meaningful data towards answering a major evolutionary question.
Dr Robert Lalonde, PhD
Department of Pediatrics
Section of Developmental Biology
Anschutz Medical Campus, University of Colorado, Aurora, CO, USA
By observing the mechanisms of embryonic development and comparing the anatomy of existing species, research groups like Mosimann’s can develop theories on how embryonic structures may have evolved or have been modified over time.
International collaboration
Collaborations with colleagues in laboratories across the country and around the world are another important part of the study. Those scientists bring additional specializations and contribute data from other models, including paddlefish, African clawed frogs, and a variant of split-tail goldfish called Ranchu, to study embryonic development.
For all the considerable work and significance of the study, the Mosimann team recognizes that it is a key step, but not the end of the journey in the debate about paired appendages.
In the abstract to their open access paper, the team say:
AbstractAnother gap filled and still no god(s) found.
The development of paired appendages was a key innovation during evolution and facilitated the aquatic to terrestrial transition of vertebrates. Largely derived from the lateral plate mesoderm (LPM), one hypothesis for the evolution of paired fins invokes derivation from unpaired median fins via a pair of lateral fin folds located between pectoral and pelvic fin territories1. Whilst unpaired and paired fins exhibit similar structural and molecular characteristics, no definitive evidence exists for paired lateral fin folds in larvae or adults of any extant or extinct species. As unpaired fin core components are regarded as exclusively derived from paraxial mesoderm, any transition presumes both co-option of a fin developmental programme to the LPM and bilateral duplication2. Here, we identify that the larval zebrafish unpaired pre-anal fin fold (PAFF) is derived from the LPM and thus may represent a developmental intermediate between median and paired fins. We trace the contribution of LPM to the PAFF in both cyclostomes and gnathostomes, supporting the notion that this is an ancient trait of vertebrates. Finally, we observe that the PAFF can be bifurcated by increasing bone morphogenetic protein signalling, generating LPM-derived paired fin folds. Our work provides evidence that lateral fin folds may have existed as embryonic anlage for elaboration to paired fins.
Tzung, KW., Lalonde, R.L., Prummel, K.D. et al. A median fin derived from the lateral plate mesoderm and the origin of paired fins. Nature (2023). https://doi.org/10.1038/s41586-023-06100-w
Copyright: © 2023 The authors.
Published by Springer Nature Ltd. Open access
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
And of course, there is absolutely no hint that the scientists thought the Theory of Evolution was less than adequate for explaining the observation or that magic done by a supernatural magician was a better explanation.
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