>They have a chart which shows the number of cell types for various classes
>of animals vs. the time in which that group appears in the fossil record.
>Here is a list (taken from the chart so there is a slight error as I had to
>measure them):
>Porifera 10 cell types 570 myr
>Cnidaria 14 cell types 560 myr
>Haemocoelic Bilaterian 30 cell types 560 myr
>Arthropoda 51 cell types 530 myr
>Echinodermata, Annelids 39 cell types 525 myr
>Agnatha 64 cell types 510 myr
>Cephalopoda 75 cell types 500 myr
>Actinopterygii 132 cell types 400 myr
>Amphibia 150 cell types 330 myr
>Diapsida 154 cell types 300 myr
>Aves 187 cell types 150 myr
>Hominidae 210 cell types 5 myr
>Clearly from this, there has been an increase in complexity throughout
>geologic time.
That depends entirely on the chosen perspective. For example, *among*
porifera, has there been an increase in complexity in the last 570 million
years? Among arthropods, has there been an increase in complexity in
the last 530 million years? Etc. [Also, don't forget to change the date of
agnatha in light of the recent fossil finds].
>The authors then constructed a computer model with 2000
>beings in which each generation of the model the beings could go up or down
>in the number of cell types. This was done by pure chance. The only limit
>was that they couldn't have negative cell types. over a number of
>generations, the maximum number of cell types increased. >>
So why haven't sponges and arthropods increased their number of cell types?
Are we to think they stopped evolving since they appeared half a billion years
ago?
Mike