The Tangled Tree: A radical new history of life by David Quammen, William Collins Lamarck’s Revenge: How epigenetics is revolutionizing our understanding of evolution’s past and present by Peter Ward, Bloomsbury
IN JANUARY 2009, at the start of the bicentenary of Darwin’s birth and the 150th anniversary of the publication of On the Origin of Species, the cover of this magazine proclaimed: “Darwin was wrong”.
This bombastic declaration related to the discovery that genes can be passed “horizontally” between distant microbial species as well as in the usual way, “vertically” down the generations of a species. “Horizontal gene transfer” (HGT) means that when we study the evolutionary relationships of microbes, we can find a thicket of connections instead of the tree-like concept developed by Darwin.
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The Tangled Tree, by science writer David Quammen, adds some intriguing new discoveries to that story. One is the fact that viral DNA, trapped in our genomes, can sometimes, as a result of natural selection, lead to key adaptations, such as the mammalian placenta. Above all, the appearance of eukaryotic cells around 2 billion years ago came about through the apparently chance fusion of two unrelated microbes rather than through inheritance.
As a result, the branch of life represented by eukaryotes – you, mushrooms, flowers and so on – is these days depicted as emerging from this cross-species event.
Nevertheless, Quammen does not nail his claim that HGT, or the origin of eukaryotes, destroys the usefulness of the tree metaphor. There is no need to fix Darwinism in this respect.
It is true that tracing the evolutionary relations between microbes has become much more complex since we discovered the extent of HGT between microbial lineages. Some arthropod species have also acquired genes from symbiotic microbes, slightly blurring their position on the tree of life. But we can still identify microbes and their relation to other organisms from their genomes. Following the recent identification of a new group, the Lokiarchaea, it neatly took up its place on one twig of the tree of microbial life, rather than being smeared all across it.
When a microbe acquires genes through HGT, only some of them survive. Natural selection retains those genes that provide an advantage and purges those that do not. While the history of a particular gene may show complex crossings between different microbial lineages, the overall relations of microbes maintain a tree-like structure.
“Darwin thought changes to an organism during its lifetime could be passed to its offspring”
The general validity of Darwin’s tree metaphor is even clearer when it comes to animals. The first version of the human genome spectacularly claimed that we have acquired more than 200 genes through HGT. We didn’t. No vertebrate genome contains a significant amount of microbial DNA. Our chromosome structure and our immune systems help stop this from happening.
One thing that Darwin definitely did get wrong was his understanding of inheritance. Like many of his contemporaries, Darwin thought changes to an organism during its lifetime could be transmitted to its offspring, hence his claim that blacksmiths’ sons have big arms.
This view is now mistakenly identified with one of his predecessors, French naturalist Jean-Baptiste Lamarck. In reality, Lamarck’s version of this conception was semi-mystical, invoking the will or desire of the young animal to change.
These basic, historical facts have escaped palaeontologist Peter Ward in his deeply misguided book Lamarck’s Revenge. He claims not only that Darwin was wrong, but that Lamarck, or the idea that he thinks was Lamarck’s, is right, as shown by modern research in epigenetics – a sexy term for gene regulation. This involves organisms responding to their own developmental stage, and factors such as environmental stressors, by turning some genes on and off, often by adding temporary chemical tags to DNA. In principle, this explains much of our physiology and growth, and why identical twins are not completely identical.
Some say that these regulatory changes can be transmitted through many generations, driving evolution. Ward goes even further, bizarrely including HGT as an example of epigenetics and arguing, without any evidence, that everything from the rapid expansion of the mammals after non-avian dinosaurs became extinct to the human propensity for warfare can be explained by epigenetic evolution.
The key stumbling block for this guff is that, in animals, there is no evidence that epigenetic effects can be inherited for more than a few generations.
For epigenetics to have a role in evolution requires a mechanism to get from changes in gene regulation in tissue to enduring changes in gametes. Darwin spent years worrying about this and came up with a completely wrong answer. Ward does not even try.
This article appeared in print under the headline “Keep calm and evolve”
