There are two features about the fossil record that are consistent with evolution.
1. Overall, there is an increase in complexity over time, for example:
- prokaryotes appeared first, then eukaryotes;
- unicellular organisms preceded multicellular;
- simpler plants and animals usually appear before more complex ones.
2. There are examples of gradual evolution:
- probably the best known is the evolution of horses from Eohippus about 50 million years ago (Mya).
These aspects are well known, and is why I had at first presumed the fossil record supported evolution (see About me).
However, to set against these, it should be noted that:
1a. There are some exceptions to the progressive increase in complexity:
- Sponges – the simplest multicellular animal life – do not appear until the Cambrian, after the Ediacaran fossils which have more complex body plans involving greater cell diversification and specialisation. (Although there are claims of fossil sponges before the Cambrian, a recent analysis argues strongly against these. )
- Bryophytes – non-vascular plants – do not appear until the Devonian period, which is later than the more complex vascular plants in the Silurian period.
1b. New groups appear abruptly, not by progressive evolution of earlier forms:
- This is most clearly exemplified by the Cambrian explosion when almost all extant animal phyla (and others which have become extinct) appear within a very short time (in geological terms) and without evident intermediates.
- The abrupt appearance of substantially new forms is accentuated by the fact that when biological innovations arise, a common pattern is for them to occur in different subgroups at more or less the same time; whereas it would be expected that a novelty would evolve in one lineage from which subgroups gradually diversified.
Even evolutionary biologists recognise that this is a problem: it is so unlikely that the mutations required for the same sort of innovation would arise independently and at about the same time.
The widespread evolutionary explanation for new forms appearing abruptly, is that preceding intermediates existed but were not susceptible to fossilisation. However, this is not a satisfactory explanation because there are many fossils of soft-bodied organisms from the Cambrian and before, as explained in the Cambrian explosion.
2. The gradual evolution that can be discerned in the fossil record is limited and can be accounted for entirely in terms of variation and selection from a gene pool present in the initial population.
- This is certainly true of the evolution of horses.
A recurring theme on this website is that much evolution can and does occur solely though mixing and selecting from existing genes, i.e. without the production of substantially new genetic material. Unfortunately, all too often this level of evolution is extrapolated to try to justify evolution that would require new genes. This unjustified extrapolation arises partly because biologists do not think carefully enough about, or look closely enough at, the different processes involved in evolution (see microevolution and macroevolution). And this also applies to evidence of evolution in the fossil record.
Notes display in the main text when the cursor is on the Note number.
1. For example, see Karl Niklas, Edward Cobb, A Keith Dunker; The number of cell types, information content, and the evolution of complex multicellularity. Acta Soc Bot Pol 83(4):337-347 DOI: 105586/asbp.2014.034
2. Jonathan Antcliffe, Richard Callow and Martin Brasier. Giving the early fossil record of sponges a squeeze, Biological Reviews (2014), Cambridge Philosophical Society, doi: 10.1111/brv.12090
Background image for banner is from https://pixabay.com/en/ammonit-fossil-petrification-2810722/ (no attribution required).
Page created November 2017.