The turtle is a mystery of evolution. Its body shape first appears in the fossil record fully formed. But how? If life evolves in small incremental steps where are all the fossils showing the development of the turtle? Quoting from the textbook ‘Explore Evolution‘:
“The very first time turtles appear, their body plan is already fully developed, and they appear in the fossil record without intermediates. Furthermore, turtle and tortoise shells contain more than 50 bony “scutes” that appear in no other vertebrate order, nor anywhere else in the fossil record. What’s more, the turtle scapula is positioned underneath its ribs and scutes, unlike any living or fossilised vertebrate. Scott Gilbert, an evolutionary biologist who works on this puzzle, says that “the turtle shell represents a classic evolutionary problem: the appearance of a major structural adaptation”. According to Gilbert, this problem is made even more difficult by “the ‘instantaneous’ appearance of this evolutionary novelty.” Because “the distinctive morphology [form and structure] of the turtle appears to have arisen suddenly.” Gilbert and his colleagues argue that evolution needs “to explain the rapid origin of the turtle carapace [shell].” They are studying turtle embryology to investigate how this might have happened.“
The image on the left is from a Times 2013 newspaper article called “At last, the hard facts on how the turtle got its shell“. I read with great interest thinking that the title was true. But I was disappointed. Nowhere in the article does it say how the turtle got its shell. The article starts its explanation with a fully formed shell – without saying how it formed. Instead the article focuses on the next steps in the turtle’s evolutionary journey. However the conclusion of the article did not disappoint:
“The team also revealed that the sequence of evolutionary events resembles the steps seen as turtle embryos develop their shells. It might not be the solution that Kipling would have dreamt up, but as Dr. Lyson [paleontologist Yale University and the Smithsonian] said: “The development data and the fossil record now align and are pointing towards the same answer.” ‘
The turtle’s evolutionary cycle resembles its embryonic growth cycle! This is an important realisation, yet I don’t think the author of the Times article or the very scientists studying the fossils grasp the implications. Even those who wrote the ‘Explore Evolution’ textbook (which is basically a criticism of Darwin’s theory) show a lack of understanding of what they are saying.
The turtle’s evolution looks like an growth cycle because evolution is a growth cycle!
If we want to know how Nature evolves life then we should look at how Nature develops life. This would lead us to look for cyclical transformative stages: tadpole to frog, sapling to tree, foetus to baby – an organism changes its body shape during its growth cycle (and often changes the environment it inhabits – tadpole in water to frog on land, chick in egg to hen on land, foetus inside a body to human on land).
What if – if only for philosophical contemplation on this windy spring morning – what if Nature did evolve life the way Nature develops life? What would we expect to find? Life begins with the seed. Well, life on Earth began with single-cell organisms… seeds? One seed is often indistinguishable from anther, but once fertilized, life begins to take its distinctive shape (as shown in the image on the left).
The same procedure appears in our fossil record: first single-cell organisms, then the Cambrian Explosion in which we find that species began to take their distinctive body shapes. In fact, the vast majority of body types that exist on Earth today began during the Cambrian Explosion.
I know that analogy is not proof. I am not offering this comparison as proof. But rather as a thought experiment. As Dr Lyson (from the Times article) said: “The development data and the fossil record now align and are pointing towards the same answer.“
It may not be what paleontologists “would have dreamt up” but the answer is that evolution is a system of growth, a process of development – as delineated and as reliable as any growth cycle in Nature.
We can’t see it because we’re in it – we can’t see the forest for the trees.
As a final note: the turtle is not the only animal to make its first appearance fully formed in the fossil record. The first bat looks very much like the modern day bat with fully functional wings (see above).
This is also true of many flowering plants which appeared during the “angiosperm big bloom”. As a National Geographic article “”The Big Bloom—How Flowering Plants Changed the World” asks: “Just when and how did the first flowering plants emerge? Charles Darwin pondered that question, and paleobotanists are still searching for an answer.”