By the time Charles Darwin published On the Origin of Species in 1859, speculations about biological “transmutation” and the relations between various groups of organisms had already been in circulation for decades. The broad outline of the geological timetable was known by 1850, and while confident assignment of the exact numbers would have to wait another century for the development of isotope dating techniques, geologists already accepted that the Earth was at least millions, perhaps hundreds of millions of years old. Fossils collected by that time were sufficient to demonstrate that dramatically different kinds of plants and animals had inhabited the Earth in past ages. Darwin’s key argument tied together observations about variability in plants and animals (both domestic and wild), and selection among those variants by breeders, or by natural conditions, to propose that the degree of organic change achievable by such mechanisms had no demonstrable limit, and that this was the ultimate explanation for the history and diversity of life, as seen both in the fossil record, and today. Darwin did not start the evolutionary ball rolling – it was already in motion – but he provided the crucial insight that tied together and explained the available data.
And yet in a real sense, Darwin no longer matters – he was in fact ignorant and even wrong about many things we now understand. For example, he had no coherent concept of the mechanism of inheritance and variation – at the time Darwin was writing, Gregor Mendel was still experimenting with pea plants in the monastery garden, and it would almost another century before genetics was properly united with evolutionary theory. Biologists today accept common descent because of several lines of evidence:
- The universality of the genetic code: with only minor variation, every organism from bacteria to humans uses the same scheme to translate DNA sequences to the proteins that drive their metabolism. Why? Because they inherited that scheme from a common ancestor.
- The fact that all organisms can be organized into a hierarchical classification scheme, based on their physical characteristics – a natural result, if they are descended from common ancestors, following a branching-tree pattern.
- The fact a similar hierarchy can be derived based on sequencing the genes of organisms – and the genetic and physical trees match closely.
- The distribution of plants and animals among various land masses. For example, the reason that large islands such as Australia and Madagascar have unique fauna is not because conditions there are somehow better suited for them (the unfortunate runaway success of introduced species like rabbits shows this is not true). Rather, it is because the current native animals are descended from whatever animals happened to be there when that area separated from other landmasses – and the timing suggested by both the local fossil record and DNA-divergence studies matches that indicated by geological evidence.
- The existence in the fossil record of transitional forms between major groups (and contrary to claims made by creationists, there is no great shortage of these). To mention merely one Canadian example: the discovery in the Arctic in 2004 of Tiktaalik, an intermediate between fish and land amphibians.
Current evolutionary research seeks to fill in the details of the tree of life, elucidate the mechanisms responsible for producing new species, and understand why organisms have the particular natures they have. But Darwin’s central insights have withstood the test of time and are at the foundation of this research program. As the geneticist Theodosius Dobzhansky put it: “Nothing in biology makes sense except in the light of evolution”.