bird-mammal common ancestor

tivol at tethys.ph.albany.edu tivol at tethys.ph.albany.edu
Wed Sep 14 10:32:33 EST 1994


Dear Matt,
	This may or may not be of help, but teleological thinking leads to the
following:  1) Directional hearing is of value to both predator and prey.  2)
Ancestors of the common ancestor were probably either or both.  3) Brain dev-
elops by addition of new parts, leaving the older structures in place (with
possible small changes in connection, but apparently essentially the same
structures).
	This leads to the thought that directional hearing developed before the
bird and mammal lines separated and was retained to the present, rather than
having re-evolved separately.  I don't know of a way to test this; however,
the existance of a nucleus in the brainstem--an older formation--in birds,
mammals and earlier forms (amphibians?) indicates retention of a useful
structure, as opposed to convergent evolution.  This evidence gets stronger
the more closely the nucleus in birds and mammals resembles that in amphib-
ians etc. considering where cells in this nucleus project, what projects to
them, etc.  It's rather like considerations involving the olfactory cortex
IMHO.
				Yours,
				Bill Tivol



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