Embryos in Deep Time: The Rock Record of Biological Development

1. Fossils, Ontogeny, and Phylogeny

1 I remember as a child being very impressed by a statement, attributed erroneously to Thomas Huxley, that claimed that if monkeys were left alone in front of typewriters, they would type by chance and, given enough time, would indeed type the entire Encyclopaedia Britannica. I had an abridged version of the Encyclopaedia in Spanish, fifteen thick volumes, so I had an idea of the extent of text involved. I read the statement for the first time in a creationism booklet, which pointed out the absurdity of the statement.1 But it made sense to me: unlikely, and yet, given infinite time, it could happen. While writing this book, I decided to investigate the matter a little and found out that this thought experiment about monkeys and typewriters has been seriously treated from philosophical and statistical perspectives and has been used in various popular accounts. In fact, this is one of the one Fossils, Ontogeny, and Phylogeny Human history is a brief spot in space, and its first lesson is modesty. Will Durant and Ariel Durant, The Lessons of History 2 / Fossils, Ontogeny, and Phylogeny best-known thought experiments, dating to a 1913 essay by the French mathematician Émile Borel. Since then it has become a popular illustration of the mathematics of probability. Apparently the likelihood of monkeys typing the Encyclopaedia Britannica or Shakespeare’s works is infinitesimally small. What is the relevance of this to a discussion about evolution and development ?2 There are two points to discuss: the length of time and the probability of evolutionary change occurring during it. Geologic time is not infinite, but it is long, very long, or deep—a good descriptor considering that it is in the depth of rocks that we can learn much about this distant past. The term deep time originated with the American writer John MacPhee’s popular account of geology titled Basin and Range. There he discussed how geologists develop a sense of the vastness of time intellectually and emotionally.3 Consideration also of the vastness of the extinct biodiversity raises a transcendent perspective , and it may be the most fundamental contribution to human understanding of the universe that geologists and paleontologists can provide. Thanks to dated fossils placed in evolutionary trees and to molecular estimates, we know that life originated at least 3.5 to 3.2 billion years ago and that multicellular life is at the very least 700 million years old.4 In the twentieth century one of the major achievements in geochemistry was the development of several methods of rock dating, based on isotopes of different chemical elements, leading to the secure establishment of an absolute time dimension of the vast history of earth and life. The evolution of biodiversity is said to have needed long periods of time. For Darwin, it was important to gather information about the antiquity of the earth and of life. He was concerned that there Fossils, Ontogeny, and Phylogeny / 3 was “enough” time for truly complex structures, such as the eye, to have evolved. The later discovery of mutations and their “randomness ” would at first glance seem to have made Darwin’s worries justified. Evolution is far from random, and monkeys in front of typewriters are not a good analogy for evolutionary processes. One of the main points made in recent books on evolution is the nonrandomness and predictability of evolution. This is not a theoretical conclusion but rather something shown empirically by the patterns seen in living phenotypes and genotypes and also in fossils. The fact that mutations in some genes have a high probability of being selected repeatedly in independent lineages facing similar environmental conditions is called “parallel genotypic adaptation.” This makes genetic trajectories of adaptive evolution predictable to some extent, leading to the reconstruction of molecular processes that most likely operated in extinct species in spite of the contingency of evolution. Development in extinct species can also be reconstructed thanks to principles that have been discovered to be shared by huge groups of species , even involving the the same developmental genes. And yet diversity, in terms of both species and breadth of form, is vast, like the deep time in which it evolved. extinCtion of most life on earth The theory of evolution provides a rational explanation for the rich biodiversity that surrounds us. Every day across the world more and more people live in cities, but even those who are rarely confronted...