evolution:dinos., birds, elephants, us...

Beverly Erlebacher bae at oci.utoronto.ca
Wed Sep 18 16:46:40 EST 1996


In article <12SEP96.20976675.0113 at VM1.MCGILL.CA> "BROWN,DAVID,MR" <BKJI000 at MUSICB.MCGILL.CA> writes:
>  On a recent trip to the NYC museum of natural history, I was struck
>by a puzzling feature of the dinosaur display: the theme of the exibit
>was one of evolution and how scientists use common physical traits in
>different species to help determine common ancestry.  My problem is
>as follows:  I have always been told, not only by watching Jurassic Park
>, that birds are decendent of certain dinosaurs and that most mammals
>are decendent of small, scurriing rodent-like creatures that appeared
>just before the dinos.dissapeared.  

Actually, early mammals showed up around the same time as early dinosaurs,
but were small and scurrying throughout the rise and fall of the dinosaurs.

>Now almost all of the dinosuars I
>saw at the museum walked on hind legs with joints pointing forward,
>i.e. knees, whereas birds walk on legs with joints that point backwars,
>i.e. elbows.  When did the first dinosaur that walked on elbows appear?
>Who were its ancestors?  Also on the mamalian front, elephants, or so
>I have been told by the good people at trivial pursuit, and the only
>animals with four knees.  How then can we and the elephants have any
>common ancestor?  No slow, evolutionary change could have brough about
>a transition from knees to elbows or vice versa.  

I think you were confused by the reconstructions of the dinosaurs and
the outward appearance of living animals.  Take the chicken as an example
of a bird.  Outwardly, they may appear to have backward-pointing "knees".
If you've ever eaten a chicken leg, you will have seen it has two parts.  The 
thigh corresponds to our thigh, and the drumstick corresponds to our shin.
The end of the drumstick is attached in a living chicken to the yellow
scaly shank at the joint corresponding to our ankle - a backward pointing
joint.  In the chicken, most of the bones homologous to our foot bones are
fused into this long narrow shank.  A live chicken or pigeon or other bird
appears to have a backward pointing "knee", which is actually an ankle,
because the ratios of the lengths of the bones are different than ours,
and everything from the ankle up is covered with feathers so the knee
joint action doesn't show clearly.

If you think about those dinosaurs walking on their hind legs, and imagine
the foot getting longer and narrower and the thigh and shin getting shorter,
you can imagine how the same bones can be modified to a more avian form.

Elephants don't have four real knees, and neither do horses have their knees
and elbows on the wrong legs, even though the most visible joint on a horse's
foreleg points forward, and the most visible joint on its hindleg points
backward.  A horse's hoof corresponds to the human middle finger nail and
the 'knee' in the foreleg is homologous to our wrist.  The elbow joint is
up close to the body.  The main hind leg joint is an ankle.

>If we are related
>would the link have to be some creature without limbs?  Is it possible
>that several mammals appeared around the world without any relation
>to one another?

No, and no.

>  Any light that could be shed on the subject would be much appreciated.
>Thank you very much.

See if you can find some library books on vertebrate or mammalian evolution. 
They often have diagrams showing the bones of different animals' legs, with
the corresponding bones shown in the same color.  It's fascinating how the
same basic structure can be changed for such radically different functions
by just changing the relative size and shape of the same bones.  Often these
books will have drawings of the embryos of different animals.  At early stages,
embryos of all mammals look amazingly alike, then the bones develop into
hands, hoofs, bat wings, seal flippers, etc., just by the relative growth
rate of different parts.

I hope this helped.  If you are interested in learning more about evolution,
I recommend the books of Stephen Jay Gould.  He is an evolutionary biologist
at Harvard who writes a monthly column in Natural History magazine.  Most
of his books are anthologies of the columns.  They are very well written
and readable.

Beverly Erlebacher
Toronto, Ontario Canada



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