Australia is renown for its coral, especially on the Great Barrier Reef. Not far from where I live is a place called Wellington Point. Just offshore is King Island, a small knoll in the ocean with a spit of sand trailing back to the mainland. At low tide, you can walk out to the island along the sandbar. My kids love collecting driftwood, catching small crabs, looking at various shells and pieces of coral that have washed up there. The sand is harsh and hard on the feet, so you have to wear shoes of some sort. It’s over a mile to the island, so you need to time your walk or you’ll end up wading back to shore, but that just adds to the sense of adventure for the kids.
We’ll come back to coral in a moment…
Although the concept of the tree of life didn’t originate with Charles Darwin, his theory of Natural Selection provided the first consistent explanation of species as they relate to each other through common descent reaching back to what has become know as LUCA, our Last Universal Common Ancestor.
The advent of genetics and our ability to investigate the weird and wonderful world of bacteria has complicated this view. With an estimated 9 million animal species on land and another 2 million estimated in the seas, the prevailing wisdom has been there’s only a few thousand archaea/bacteria species, a number that is so pitifully low as to serve as a red flag that’s far too low to be accurate. Recent estimates have revised this into the millions. In reality, given the vast evolutionary dominance of archaea and bacteria over billions of years, even this figure may be conservative. And this is where things get interesting when it comes to the tree of life.
Prokaryotes (bacteria & archaea) use asexual reproduction, allowing them to transfer genes horizontally with each other rather than one-way down through the generations. They are, for the lack of better word, incestuous. Rather than a simple, clear, tree structure, various genes follow a number of pathways over time. It seems the tree structure only applies to eukaryotes (animals, plants, etc), and then only after a certain point in our evolutionary history, a point probably somewhere around a billion years ago.
Recently, an article was published title Life after Darwin, suggesting that the concept of the tree of life was outmoded and that “Darwin’s theory of evolution is more a hindrance than a help, because it has become a quasi-theological creed that is preventing the benefits of improved research from being fully realised.”
The problem with this view is that it is largely ignorant of “Darwin’s theory of evolution,” and to me, at least, seems to be yet another grandstand attempt at sensationalizing science to grab headlines.
Darwin-bashing appears to be a common pastime these days, perhaps not always consciously, but it seems Darwin is an easy target. Every couple of months there’s an article proudly proclaiming Darwin got it wrong, or we need to move beyond Darwin, etc. I’d like to suggest we need to understand Darwin first, as even now, over a hundred and fifty years after the publication of On the Origin of Species, there are gems in his work that are largely misunderstood/overlooked even among the scientifically literate. In this particular case, there’s a fundamental misunderstanding of what Darwin proposed.
The tree of life is an analogy. The way in which life has evolved on Earth is comparable to that of how a tree branches out, but like all points of comparison, the analogy will fail if extended beyond what was intended. Darwin himself recognized this comparison was limited in how it described the natural progression of species over time and preferred to compare the evolution of life on Earth to a coral head.
The tree of life should perhaps be called the coral of life, [its] base of branches dead; so that passages cannot be seen.
Life and Letters of Charles Darwin – Volume 1, pg 368
Darwin’s comparison is quite astute in that coral branches out in a similar manner to a tree, but with more regular, repeating structures and patterns, with each living polyp growing upon the dead skeleton beneath it in the same way as each species builds upon those parent species that preceded them.
Remarkably, Darwin’s coral analogy holds true where the more literal tree analogy breaks down in the light of modern genetics and the horizontal transfer of genes in bacteria/archaea. Corals, with their thick bases entwined upon themselves are a good likeness for the bulbous relationships in the bacterial world. Corals only slowly form diverging sections that split into trunks, reaching out into limbs and branches.
A sponge coral, like this one with a vase coral growing up within it, is a good representation of the microbial world, as just like the tree of life, the microbial species we see around us today are built upon the extinct parent species of yesterday, only there may be multiple parent species.
Representing something as complex as the evolution of life on Earth with an analogy is always going to be difficult. There are always going to be limits to the comparison, but such analogies do increase our preception of how evolution has progressed according to Natural Selection, and so are entirely valid.
Darwin’s coral analogy was perhaps more insightful than he realized at the time. In the final analysis, we are quite likely to come up with something similar to this hybrid example, where a branching coral tree has grown up with a coral sponge surrounding it. Yes, the analogy is limited in that it only represents one form of genetic progression when different genes can follow different paths, but overall it is a good approximation.
To suggest that “Darwin’s theory of evolution is more a hindrance than a help” is both absurd and asinine and shows a distinct lack of understanding as to what Darwin actually proposed.
Natural Selection holds true for descent with modification in the animal/plant kingdom as well as horizontal genetic transfers within prokaryotes. In both cases, species are subject to survival of the fittest. The tree of life is an appropriate analogy for the plants and animals we are so familiar with, while the complexities introduced by our modern understanding of prokaryotes fits with Darwin’s proposed coral of life.
More than a century has passed and Charles Darwin’s works are as relevant as ever.