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04/19/2006
A Couple of Links
I was stunned by the overwhelming response to last week’s
column on crows. OK, I only got two e-mails on the column –
one from Charles K, the other from Patti D. Apropos of the
avian theme, Charles points out that some birds are mostly
walkers while others are hoppers. He wonders why and observes
that the smaller (nervous, cautious) birds are the hoppers while
the larger “lumbering” birds such as crows are walkers. I would
like to add a third category, the “skitterers” that I’ve observed on
my walks on the beach on Marco Island. Even after many years
of observing the skitterers I can’t understand how these tiny
shorebirds can move so fast. Charles suggested that I look into
this avian locomotion question. The whole question of
locomotion in all kinds of animals certainly is a subject worthy
of further consideration.
Actually, this month has seen the publication in the April 6 issue
of Nature a major find related to the very essence of locomotion
on land. This was the discovery in northern Canada of fossils of
a 375-million-year-old fish the discoverers have called Tiktaalik
roseae, after an Inuit term for “big freshwater fish”. Ted
Daeschler of the Academy of Natural Sciences of Philadelphia,
Neil Shubin of the University of Chicago and Farish Jenkins Jr.
of Harvard authored the paper. You may have seen Daeschler
interviewed on the PBS NewsHour with Jim Lehrer.
You may also have seen pictures of an artist’s conception of the
4 to 9-foot Tiktaalik in the media. Why all the fuss over a fish?
Well, Tiktaalik was no ordinary fish. It had a head like a
crocodile but the jaws of a fish. Its eyes were on the top of its
head and, unlike your normal fish, it had a neck. With a neck,
Tiktaalik could lift its head. In addition, the bones in its fins
were more like the bones in arms and forearms and there was
even a hint of a wrist and a hand. The construction of the fins
was such that they weren’t all that great for swimming but were
powerful enough to have been pushed Tiktaalik out of the water
onto the shore.
In other words, this Tiktaalik critter was not much of a swimmer
nor was it very agile if it dragged itself up on shore. If you’re
thinking missing link, you’re not alone. Tiktaalik is unlike other
missing links that have been found linking the time when
animals emerged from the sea and started walking on land.
Those other fossils have been of animals that were obviously
either more fish-like or more tetrapod-like. (Tetrapods are
animals with four limbs; we’re tetrapods.) Tiktaalik, on the other
hand, seems caught between being a fish and a tetrapod, barely
able to crawl on land; hence the excitement over what appears to
be the finding of a true missing link, capturing life emerging
from the sea onto the land.
Finding missing links is not for the faint hearted. Tiktaalik was
not found until the last of four summers the researchers spent
poking around up there in the wilds of northern Canada. With
severe wintry weather setting in, they had to leave part of their
biggest fossil behind. Fortunately, they managed to bring
enough fossil material back to keep them quite busy.
Weather conditions were quite different for Tim White and the
multinational team of which he is a co-leader in the Middle
Awash area of the Afar desert in Ethiopia. White calls the
Middle Awash “the world’s best window on human evolution”.
With layers of sediment nearly a mile thick, the fossils uncovered
in this area cover some 6 million years of evolution dating from
the time that our ancestors split off from the chimpanzees down
to humans as we are today. Last week, in the April 13 issue of
Nature, White and his co-authors reported finding 4.1 million-
year-old fossils that bridge a million-year gap in the fossil record.
Another missing link found.
Chances are you’ve heard of “Lucy”, the skeleton of a small,
three and a half foot tall adult that was found back in 1974 in the
Middle Awash. Tim White was also involved in the analysis of
Lucy’s remains. Lucy was later classified as belonging to the
hominid species Australopithecus afarensis (let’s call it Austry
for short). Since we’re concerned about locomotion in this
column, we should note that Lucy and her earlier Austry
ancestors were walking on two legs back about 3 to 3.6 million
years ago. (Quite an advance over the clumsy Tiktaalik, a
possible ancestor hundreds of millions of years earlier.) Other
earlier Austry species preceded Lucy, back to about 4 million
years ago.
The Middle Awash also has yielded fossils of earlier hominids in
the category known as Ardipithecus (let’s call it Ardy for short).
There was a gap in the fossil record and it was not known for
sure whether Austry evolved from Ardy or from some other
species. This was the million-year gap. The most recent find
included teeth and jawbones from 8 individuals living 4.1 million
years ago. These individuals have been identified as primitive
members of the Austry clan, fitting neatly between Ardy and the
later Austry. As Tiktaalik has provided the link between sealift
and land life, so the latest fossils from Ethiopia are believed to be
the missing links between Ardy and Austry.
Being currently in the process of getting a crown on one of my
teeth, I was naturally interested in a key difference between Ardy
and Austry. Ardy had small teeth and hence could only eat
relatively soft foods. Austry developed large teeth and could
handle much rougher foods with lots of fiber. This ability to sup
on a wider variety of foods probably served Australopithecus
well when times were tough and the food supply was limited.
Australopithecus is thought be the species from which our own
brand of Homo species arose so I suspect we owe those larger
teeth a debt of gratitude.
Of course, I can’t let these two missing link stories go by without
noting that they just add more weight to the validity of
Darwinian evolution. Just as Tiktaalik and his like paved the
way for mammals and the dinosaurs so did the dinosaurs give
rise to the birds. As my wife and I pulled into our driveway last
Saturday there was a wild turkey in our backyard, the first we’ve
seen on our property and only the second time I’ve ever seen one
in our town. I guess that crows are not the only birds that are
migrating from the country to urban areas. Incidentally, the
turkey was a walker, not a hopper.
Finally, I mentioned the e-mail from Patti commenting on the
crow column last week. It turns out that she has an African grey
parrot. This was revealed when I mentioned that, if I were
younger, I would get an African grey and see for myself how
intelligent they can be. (In past columns, I’ve talked about Alex,
an African grey whose exploits have put him on TV and are
astounding to me.) Patti’s parrot is obviously quite intelligent,
with an extensive vocabulary and he often uses the correct
phrases in context, such as asking about broccoli, yogurt, carrot
or apple when he hears Patti in the kitchen. He also imitates the
ravens in her yard and whistles the theme from the Andy Griffith
show. Hey, that’s more than I can do, not having watched that
old TV series.
Allen F. Bortrum
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