There is interesting data on brain size that seems to show its
increase as a relatively steady process.
I quote from my great filter paper:
>Finally, brain size relative to body size has been increasing somewhat
>steadily for both mammals and birds ever since the mass extinction of
>65 million years ago (most likely also caused by an external event
>such as an asteroid) eliminated the dinosaur competition [Russell 83,
>Jerison 91]. ...
>
>Harry J. Jerison (1991) <i>Brain Size and the Evolution of Mind</i>,
>American Museum of Natural History, New York. ...
>
>Dale A. Russell (1983) "Exponential Evolution: Implications for
>Intelligent Extraterrestrial Life" <i>Adv. Space Res.</i> 3(9):95-103. <p>