How an immortal jellyfish could end evolutionary theory as we know it

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Turritopsis dohrnii, the ‘immortal jellyfish’. Image by Dr. Karen J. Osborn via Wikimedia (CC0 1.0)

Almost all organisms grow old (i.e. physically deteriorate over time), a process that eventually results in death. Biologists call this age-related decline ‘senescence’. We know only a few exceptions. The ‘immortal’ jellyfish periodically reverts to an earlier stage in its life-cycle, like a butterfly turning back into a caterpillar.

This life-cycle reversal has been observed in another species of jellyfish, . A flatworm called Schmidtea mediterranea appears to be able to regenerate itself indefinitely. Another sea creature called seems to not age at all. Lobsters don’t show obvious signs of deteriorating with age, but they never stop growing. Since their shells don’t expand, they have to discard them and grow larger ones. Eventually, it just takes too much energy to grow a bigger shell, and lobsters succumb to exhaustion, if nothing else kills them.

How do these species defy the aging process? In brief, their cells are able to self-repair or regenerate indefinitely. All species can self-repair, but apart from rare exceptions like those above, the ability to renew damaged cells weakens over time, due to the declining production of enzymes such as telomerase.

One of the tenets of Darwinism is that all biological traits evolved, which entails that senescence too is a product of evolution. Sure enough, there is a vast scientific literature on the ‘evolution of senescence’, with competing theories on why natural selection might favor a limited life span in almost all species. The jury is still out on that question, but the elephant in the room has been largely ignored.

Namely, the implicit assumption that there was possibly a time when there was no senescence. What are the implications for Darwinism if senescence was, on the evolutionary scale, a recent development (say, less than a million years old)? That is, if until very late in evolutionary history, more species than today, and perhaps even all species, did not undergo senescence?

Let that thought sink in for a while. Because it’s so alien (outside of fantasy novels) that we may not immediately grasp all that it entails. To put it another way, it is possible that until the relatively recent past in Earth’s history, all organisms were immortal. They could have died from accidents, disease, or predators, but not old age. So barring those other causes of death, some creatures may have lived hundreds, thousands, perhaps millions of years.

Why is this possibility a threat to Darwinism? Because every species goes through various life stages as they mature, and a species’ body plan may alter radically from one stage to the next. The classic examples are the tadpole and adult stages of the frog, and the caterpillar and mature stages of the butterfly (image below).

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Caterpillar to pupae to butterfly metamorphosis. Photos by Nick Hobgood via Wikimedia (CC BY-SA 3.0)

If one had not known these were the life stages of the same organism, one would naturally assume they were different, completely unrelated, species. The title of Darwin’s classic book was , and he explained the appearance of different ‘species’ in each layer of the fossil record in terms of natural selection and random mutation, just as Neo-Darwinism does today.

Now, imagine if species lived for thousands, perhaps millions of years and underwent multiple life stages, each as radically different from the other as the tadpole from the frog, or the caterpillar from the butterfly. Then periodically, a mass extinction event (say, an asteroid strike) kills off some members of a species at a particular life stage, while other members grow through to the next stage. What would that look like in the fossil record? It could look precisely like what we observe today, if we replaced ‘species’ with ‘life stage’. In which case, natural selection and random mutation would have little to do with the appearance of radically new body plans in the fossil record, anymore than with the metamorphosis from tadpole to frog, or caterpillar to butterfly.

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Skulls of Triceratops (A) and Torosaurus (B). Torosaurus has a larger frill with two holes in it. Photos by Nicholas R. Longrich, Daniel J. Field via Wikimedia (CC BY 2.5)

But surely, scientists couldn’t mistake two life stages of the same species for two different species? For over a century, paleontologists believed that Triceratops and Torosaurus were two different species of dinosaur. Until about 2009, when MSU paleontologists John Scannella and Jack Horner came to the conclusion that Triceratops was actually the juvenile stage of Torosaurus. Suspicions arose when they found Triceratops skulls of various sizes, but only large Torosaurus skulls. Only a close study of differences in the skulls of Triceratops revealed the possibility that they were at different stages of turning into Torosaurus. However, Scannella and Horner’s hypothesis has not been universally accepted. Which goes to show how tricky it can be to distinguish between life stages and species in the fossil record.

Remember that timeline chart in evolution textbooks (image below), the one that shows a fish at the bottom, then slightly above it, a fish with leg-like fins, followed by a land creature with legs, etc? How do we know those are different species, and not life stages of the same species, possibly spanning thousands, if not millions of years? Which hypothesis seems more plausible, if senescence was not an issue?

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A cladogram of the evolution of tetrapods showing the best-known transitional fossils. From bottom to top: Eusthenopteron, Panderichthys, Tiktaalik, Acanthostega, Ichthyostega, Pederpes. Image by Maija Karala via Wikimedia (CC BY-SA 3.0)

Compare the chart above (illustrating ‘the evolution of tetrapods’) with the one below, of the life stages of the green frog. Note that the tadpole below has gills, like the fish above, while the adult frog is air-breathing, like the land tetrapods above.

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Life cycle of the green frog (Rana clamitans). Image by LadyofHats via Wikimedia (CC0 1.0)

Of course, the life-stage hypothesis raises the question of how complex species with multi-stage life cycles could have arrived early in the fossil record. However, that’s a problem for Darwinism too, after the discovery of the Burgess Shale, close to the bottom of the fossil record, yet filled with complex organisms (image below), while the Ediacaran layer beneath it (last image) is relatively sparse, with no obvious transitional forms to the layer above.

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Biota of the Burgess Shale, one of the earliest fossil layers, yet filled with complex organisms. Image by M. Alan Kazlev via Wikimedia (CC0 1.0)
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Fauna of the Ediacaran. Image by Foolp via Wikimedia (CC BY-SA 4.0)

A possible objection to the life-stage hypothesis is that dinosaurs lay eggs that hatch baby dinosaurs, not some earlier life stage that could have been mistaken for another species. The same goes for most species today, with a few exceptions such as frogs, butterflies, and some jellyfish. But the evidence for what hatched from who gets murkier the further back we go in time. It’s possible that when there was no senescence, organisms could have had offspring at more than one stage in their life cycle, with the babies resembling the parent at that stage in the parent’s life (by analogy, as if tadpoles gave birth to tadpoles, and frogs to frogs). Since the advent of senescence would have cut out or truncated most life stages, it makes sense that post-senescence species would only be fertile in one life stage.

A further objection to the life-stage theory is that it offers no naturalistic explanation for the origin and evolution of life. On the question of origin, Darwinism fares no better. Attempts to reproduce the postulated earliest stage of organic evolution, the development of living creatures from non-living matter, have only succeeded in producing biochemicals, not organisms capable of metabolism, self-repair, and replication. Even the most radical Darwinian concedes that evolution is not an entirely random process, but is heavily structured by natural laws. The life-stage hypothesis suggests that life is possibly much older than we currently think, if we posit a microscopic but genetically complex ‘spore’ stage predating the Ediacaran. Which would make the question of life’s origin and evolution really a question about the origin and evolution of the universe.

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Ben writes on the theory and social science of communication, and anything else that comes to mind

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