ethan@utastro.UUCP (Ethan Vishniac) (11/06/84)
[]
The following are responses to those comments in L. Bickford's
latest article which are specifically directed towards me, followed
by some general comments.
>> However, the best reading of the available evidence is that the common
>> ancestors of crocodiles and chickens lived in the Early Triassic,
>> whereas the lineage of snakes split off in the late Permian.
>What evidence? Has a missing link been found? Or is this an
>interpretation forced by the presumption of evolution?
This comment is strangely irrelevant to the argument I was addressing
here. Allow me to clarify. A creationist (I believe it was R. Miller)
noted that the biochemical evidence would lead an evolutionist to conclude
that chickens and crocodiles were more closely related to each other than
either was to lizards and snakes. However, *he claimed* evolutionists
would have expected that reptiles (such as crocodiles and snakes) ought
to be more closely related to each other than either one is to birds.
That is, he was asserting that evolution made a prediction on the
biochemical similarities of species which was clearly wrong.
My comment was that based on the fossil record, *and assuming evolution*,
people predicted exactly the relationship that the biochemists
have found. This therefore counts as a successful prediction that
followed from the theory of evolution. I'll return to this later.
The comment about "missing links" is one that I'm continually
confused by. What is a "missing link"? Presumably something that
shares some of the characteristics of the forms it is "transitional"
between. By that definition an overwhelming number of "missing links"
have been found. It is as though one were to assert that the
interval between 0 and 1 was not continuously filled with numbers, but
that there were "missing links" between the two. Whenever this
proposition is confronted with intermediate numbers ( e.g. 0.5, 0.75)
the argument is modified to include the discovered numbers as
delimiters of the missing intervals and the claim is repeated that
there has been no progress towards uncovering the "missing links".
>> After a species has appeared in the fossil record, its subsequent
>> fossil record depends on the success of the species, the likelihood
>> that the environment in which it lives are conducive to fossil
>> formation, and the element of luck involved in its discovery.
>These seem more like apologies for inadequacies of the record. By these
>same measures, a creature could have existed well *before* the fossil
>record indicates.
Yes, life would be easier if one could just look up the answer
in a book. In real life we need to reason from the evidence.
Sometimes that evidence is comlete and sometimes not. In the fossil
record some lineages are recorded in great detail and others are
not. An individual creature might have evolved earlier than its
earliest traces, but the emergence of related groups, sharing common
features makes an unmistakeable trace in the fossil record. In order
for a lineage to be *much older* than it appears it needs to have
been consistently unsuccessful throughout its history, and to have no
successful relatives. For obvious reasons this is not expected to happen.
Thus human beings might be older than the earliest human skeletons, but
there is no getting around the hypothesis that we are descended from
the great explosive radiation of the great apes without abandoning
evolution. I should probably note that humans have one advantage in
terms of the fossil record. We make stone tools. These are easily
preserved. There are none found before about 2.5 million years and
these are comparatively crude. There are no hominid skeletons between
2 and 3 million years old, but older skeletons are classified as
Australopithecus Afarensis (not Homo) because their brain capacity is
barely larger than a Chimpanzee's.
>> To be more specific, about 26 million years back we start finding
>> fossils of large primates with characteristics reminiscent of the
>> great apes and of humans.
>Is this a hypothesis or a specific reference?
This is a specific reference to Aegyptopithecus, except I got the
date wrong. It's actually about 30 million years old. In evolutionary
theory it must necessarily be either an ancestor of the great apes and
human beings, or a close relative of that ancestor (which is just as good
for our purposes). The real most recent common ancestor is usually taken
to have lived more recently, but Aegyptopithecus provides a firm upper
bound. The fossil record for the upper primates is confusing and
it is nearly impossible to assign the living representatives of this
group to specific fossil groups. The problem appears to be that the
typical habitat of the great apes has been the tropical forests.
These habitats are not very favorable for fossilisation. As a result,
when these forests began to shrink (about 10 million years back) the
fossil record for the great apes became scanty. Biochemical evidence
suggests an ordering of relationships between man and the great apes
which consists of individual lineages splitting off from the one that
lead to man. That is that Chimps and humans split recently. Gorillas
diverged somewhat earlier. Orangs slightly earlier than Gorillas, and
Gibbons earlier still. Note that since the biochemical evidence has
matched the fossil record in other, better documented cases, that this
constitutes a prediction of the kind of fossils we will find.
>> The [Richard] Leakey quote is both (probably) accurate and completely
>> bogus. I believe that he said that, but he was obviously speaking
>> about the details of *human* evolution. Humans are an example of a
>> line which has been poorly preserved in the fossil record. The line
>> from Homo Erectus to Homo Sapiens is very well understood. The
>> connections to be made back from that are tentative with the precise
>> relationships between different specimens sometimes uncertain. It is
>> being deliberately obtuse to confuse that with the question of whether
>> or not human beings evolved.
>Not obtuse, just skeptical of the basic assumption rampant through the
>quote: that evolution actually occurred. Evolution is being assumed in
>order to prove evolution.
The obtuseness I was referring to was your abuse of Leakey's quote
to suggest that he was skeptical of the theory of evolution when he
was actually discussing the detailed history of man's recent evolution.
One statement that creationists and evolutionists seem to agree on
is that an important test of a scientific theory is that it explain the
observations in an economical and natural way. Creationists feel that
their theory meets this test because the same creator is invoked to
explain everything. What could be simpler? I think this totally
misses the point. Simplicity in explanation should include providing
a theory which relates the known facts and gives us firm expectations
about the results of future experiments. Consider the history of
the theory of evolution. When it was proposed it was meant to be
an explanation of the ordering of fossils in the *previously*
derived geological column. This entailed some clear expectations
about future observations. They were:
1. That the future discovery of fossils would fit into the
evolutionary pattern. This is a tight constraint in some
ways and loose in others. Gross anachronisms are clearly
forbidden, but when a lineage is poorly known than almost
any discovery of intermediate forms is consistent with
evolution. However, as the fossil record becomes more
clearly known the available constraints increase.
2. That the sequence of rocks in the geologic column would
prove to be a true sequence of relative ages when and if
a reliable method of determining ages was established.
3. That the relationships deduced from the fossil record would
be validated by any other means discovered to measure
the degree of kinship between species.
At the time the theory was proposed it was expected that
only the first method could ever be used to check the theory.
The discovery of radiometric methods of decay provided a method
for checking the theory using method number 2. Finally, the
recent progress made in biochemistry has provided a means of
checking the theory using method number 3. In all cases the
predictions made by the theory of evolution have been validated.
Now it is worth noting that no one has ever proposed (on this
net to my certain knowledge) any tests of predictions of creationism.
I think this is because creationism, not being a scientific theory,
has no means of making testable predictions. If one were to propose
a testable theory of creationism then it would be reasonable to demand
that it amass as impressive a record as the one quoted above for the
theory of evolution.
This has gotten awfully long, and I have work to do. I think
I'll leave it to others to answer the rest of the article.
"I can't help it if my Ethan Vishniac
knee jerks" {charm,ut-sally,ut-ngp,noao}!utastro!ethan
Department of Astronomy
University of Texas
Austin, Texas 78712