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Sex organs started out as extra legs on vertebrates?

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In placoderms, some researchers say, 430 to 360 mya:

Placoderms were the world’s first megapredators, with ancient behemoths such as Dunkleosteus reaching the same size as today’s Great White Shark.

They were also the first vertebrate animals on Earth to have a complex form of sexual reproduction – copulation – where males fertilised the females internally rather than just spawning in water.

To achieve this, the males bore bony paired structures – the claspers – which were previously thought to be similar to the cartilaginous claspers of modern sharks.


Unlike the claspers of modern sharks and rays that are a part of the paired pelvic fins, the claspers and female basal plates in placoderms were not at all connected to that fin.

Instead they developed as an extra pair of limbs further down the body.

This discovery implies that the first vertebrates did not conform to the typical tetrapod-like pattern of having just two pairs of paired limbs (arms, or pectoral fins, and legs, or pelvic fins) that typifies the basic jawed vertebrate body pattern from fishes through to mammals. More.

In that case, it was convergent evolution that caused the placoderm claspers to seem like those of sharks.

That said, the claim that some vertebrates were six-legged will (and should) give many pause for thought. Was it ever seen elsewhere?

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Dang, and I was really getting into this evolution thing! ;-) -Q Querius
Remember, you heard it first here!
Yes, never heard it before. Dionisio
Actually, I wanted to pursue the idea that leg and fin appendages evolved out of versatile sex organs. As early organisms were evolving, sexual polymorphism must have been a precursor to sexual dimorphism. Thus, it's likely that these early appendages were a veritable Swiss army knife of possible pairings. Later, as evolution began to narrow sexual organs into dimorphism, the other, vestigial sex organs must have found an evolutionary pathway to ever-greater specialization as either aqueous or terrestrial locomotion. Remember, you heard it first here! -Q Querius
Limb skeletal pattern relies heavily on graded Sonic hedgehog (Shh) signaling. As a morphogen and growth cue, Shh regulates identities of posterior limb elements, including the ulna/fibula and digits 2 through 5. In contrast, proximal and anterior structures, including the humerus/femur, radius/tibia, and digit 1, are regarded as Shh independent, and mechanisms governing their specification are unclear. Here, we show that patterning of the proximal and anterior limb skeleton involves two phases. Irx3 and Irx5 (Irx3/5) are essential in the initiating limb bud to specify progenitors of the femur, tibia, and digit 1. However, these skeletal elements can be restored in Irx3/5 null mice when Shh signaling is diminished, indicating that Shh negatively regulates their formation after initiation. Our data provide genetic evidence supporting the concept of early specification and progressive determination of anterior limb pattern. From http://www.cell.com/developmental-cell/abstract/S1534-5807(14)00153-1
Very simple, piece of cake, isn't it? Dionisio
Regeneration of a lost appendage in adult amphibians and fish is a remarkable feat of developmental patterning. Although the limb or fin may be years removed from its initial creation by an embryonic primordium, the blastema that emerges at the injury site fashions a close mimic of adult form. Central to understanding these events are revealing the cellular origins of new structures, how positional identity is maintained, and the determinants for completion. Each of these topics has been advanced recently, strengthening models for how complex tissue pattern is recalled in the adult context. http://www.cell.com/developmental-cell/abstract/S1534-5807(14)00206-8
Very simple, piece of cake ;-) Dionisio
Fins are not legs (though tetrapod legs evolved from pectoral and pelvic fins).
That is the untestable propaganda, anyway. It isn’t science. Joe
Fins are not legs (though tetrapod legs evolved from pectoral and pelvic fins). Tat is the untestable propaganda, anyway. It isn't science.
As more of my fellow EEE and CS colleagues enter the biological research field, some folks will have a hard time trying to write things that are hard to demonstrate, because they'll have to answer many detailed questions. We enjoy asking questions. Non stop ;-) In EEE and CS we can't get away with hogwash. Better watch out. EEE = Electronics and Electrical Engineering CS = Computer Science Dionisio
Evolving Hox Activity Profiles Govern Diversity in Locomotor Systems ...the generation of limb-projecting lateral motor column (LMC) neurons in mice relies on a transcriptional autoregulatory module initiated via transient activity of multiple genes within the HoxA and HoxC clusters. Repression of this module at thoracic levels restricts expression of LMC determinants, thus dictating LMC position relative to the limbs. This suppression is mediated by a key regulatory domain that is specifically found in the Hoxc9 proteins of appendage-bearing vertebrates. The profile of Hoxc9 expression inversely correlates with LMC position in land vertebrates and likely accounts for the absence of LMC neurons in limbless species such as snakes. Thus, modulation of both Hoxc9 protein function and Hoxc9 gene expression likely contributed to evolutionary transitions between undulatory and ambulatory motor circuit connectivity programs. DOI: http://dx.doi.org/10.1016/j.devcel.2014.03.008
likely contributed? how? where is the detailed explanation of that hypothetical change of mechanisms? Dionisio
Note in the quoted text in post # 12 that apparently several developmental mechanisms appeared at the same time. How did that happen? Well, scientists still try to figure out how those mechanisms work interrelated, hence they don’t seem to have much spare time to work on origin issues.
Do they care? Apparently many don't. Dionisio
More development-related mechanisms described in this thread: https://uncommondesc.wpengine.com/evolution/a-third-way-of-evolution/ (see over 100 posts starting at # 9) Dionisio
Note in the quoted text in post # 12 that apparently several developmental mechanisms appeared at the same time. How did that happen? Well, scientists still try to figure out how those mechanisms work interrelated, hence they don't seem to have much spare time to work on origin issues. Dionisio
Limb innervation is established by opposing Hox-dependent activities. In this issue of Developmental Cell, Jung et al. (2014) show that Hoxc9 restriction of Foxp1, high levels of which specify limb-innervating motor neurons, first appeared in vertebrates concomitantly with paired appendages. Spatial control of this activity shapes neural networks controlling locomotion patterns. DOI: http://dx.doi.org/10.1016/j.devcel.2014.04.014
first appeared? how? Note that the above referred article just attempts to describe part of the entire development mechanism for limbs. Dionisio
To be fair, the fossil record does include some well known examples of transitional forms that correspond to the first definition, such as the fishapods (presumably documenting the emergence of tetrapods from lobe-finned fish) and feathered dinosaurs (presumably the evolutionary ancestors to birds). Even so, when we consider the details of these high-profile examples, some rather uncomfortable problems surface for the evolutionary paradigm. For example, both the tetrapod and bird evolutionary sequences suffer from what paleontologists call a temporal paradox, in which the transitional forms (i.e., fishapods and feathered dinosaurs) appear in the fossil record after the evolutionary end products (i.e., tetrapods and birds, respectively). The bottom line is that when the fossil record is considered in its entirety, the patterns found therein find perfect explanation from a creation model vantage point, rather than from an evolutionary one. For a detailed discussion of the temporal paradox and other problems associated with the evolutionary models for tetrapod and bird origins, check out the following articles: Read more on this in http://www.reasons.org/articles/q-a-are-there-transitional-intermediates-in-the-fossil-record
How does the existence of fishapods fit a creation interpretation of life’s history? For one, these animals were perfectly suited to live at the water’s edge. Also, the mosaic of fish and tetrapod characteristics possessed by the fishapods signifies design. Human designers frequently design objects and systems that blend properties and features of different systems. If humans create mosaics, why wouldn’t a Creator do the same? These fascinating creatures, well-designed to exploit their environment, make sense from a creation perspective. Conversely, overlapping fishapods that appear in the “wrong” order in the fossil record indicate Darwin’s long-sought “innumerable transitional forms” still don’t exist. Read more on this in http://www.reasons.org/articles/tetrapod-transitions-evidence-for-design
Piotr: Are you sure you are a linguist, and not a biologist? :) gpuccio
Fins are not legs (though tetrapod legs evolved from pectoral and pelvic fins). There were some early vertebrates with even more than three pairs of fins on the underside. Acanthodians (the "spiny sharks") are notorious for that. Brochoadmones milesi from the earliest Carboniferous had six additional pairs of prepelvic spines supporting "finlets" (a total of eight pairs), and the Early Devonian Euthacanthus gracilis had 4-6 (usually five) intermediate fin spines. Piotr
tjguy, Sorry buddy, but you don't understand evolution ;-) Dionisio
Actually, 6 legs is not so out of the question to me. The genetic information for legs is already present. Nothing new needs to be produced. Sometimes people are born with 6 fingers, right? However, the change from a leg to sexual organs is pure hoopla! How in the world could such a change take place? It makes for a nice story and seems to be enough to satisfy most Darwin supporters, but the details are missing. Heck, no one even knows if such a change is possible! Can you imagine the changes that would be necessary in those legs to turn them into sexual organs? Where is the new software, proteins, and genes necessary to form and control these organs going to come from? What good would a half formed anything be? I mean you have the male organ, but it has no urethra? Or it's not connected to the sperm? I don't know, but it sounds like a real stretch to me! tjguy
Sometimes mine does seem like a third wheel. Mung
They obviously evolved away, of course. There's an overwhelmingly rich fossil record of exactly these changes simply waiting to be discovered! "This is a painting of a cow." "Where's the grass?" "The cow has eaten it." "Where is the cow?" "After eating the grass, the cow has left." -Q Querius
It's an interesting hypothesis, to be sure, but six legs on a vertebrate? One would like to see additional instances. News
Actually, this makes much more sense than most announcements from the Darwin crowd. I will have to say that there's just as much evidence that legs actually started out as highly mobile sex organs, and that there was evolutionary competition between mobility and reproduction---sort of equivalent to the guns and butter dichotomy in macroeconomics. Sorry, couldn't help myself. ;-) -Q Querius
I'm just glad they're not on fingers. I have a tough enough time typing as it is. ppolish

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