Encyclopedia of Evolution.pdf - Online Reading Center

chapter 10. On the imperfection of the geological record
I have already explained why it is that we do not today see
numerous intermediate forms between species: Natural selection
favors the forms that are most distinct from the parents,
and from one another, in a group of newly evolving varieties.
Now I will explain why these innumerable intermediate forms
seem to be absent from the fossil record as well. “… this, perhaps,
is the most obvious and serious objection which can be
urged against [my] theory.”
The geological record is highly imperfect; indeed, it
could hardly be otherwise. We know that different breeds of
pigeon were bred from a single ancestral species; yet there is
very little if any record of this in the fossils.
Go look for yourself, along a shoreline, and see the erosion
that is there taking place, and think about how slowly
and irregularly erosion is occurring there, to be deposited in
the shallow oceans nearby. Yet this erosion has occurred,
in the past, enough to produce a total of almost 14 miles of
depth in geological deposits, and these 14 miles represent
only the period of time in which deposition was occurring,
during times of the subsidence of the crust; at least half of the
time, the crust was rising, causing erosion instead of subsidence.
After all, if you have deposition in one location, you
must have erosion somewhere else. You will not find all 14
miles in any one location; this figure is calculated by adding
up the deposits in one location that correspond to those of
another location [a procedure developed by earlier geologists;
see Smith, William].
During my journeys I saw, in one location, a mass of
conglomerate rock more than 10,000 feet thick. This rock
was formed of rounded pebbles imbedded in metamorphic
rock that had been transformed by heat; this conglomerate
rock, therefore, was formed from earlier rocks. How old the
Earth must be! Just how long is a million years? Some professors
have demonstrated this to their students by making a
mark, one-tenth of an inch in diameter, on a strip of paper
83 feet 4 inches long; the mark represents a century, the strip
of paper a million years! Yet the Earth has existed many millions
of years.
Here are just some of the reasons that the geological
strata are imperfect as a record of past life on Earth:
1. Only a few places on the Earth have been thoroughly
explored by geologists.
2. The only organisms that are preserved as fossils are those
that die:
• during periods of deposition. Those that die during
periods of erosion are unlikely to get covered with sediments
and preserved.
• at a time and place where deposition is occurring rapidly
enough to bury them before they decompose. In most
parts of the ocean, sediments are not accumulating; this
is why most of the ocean is blue.
• underwater, or where they can be transported to a riverbed,
delta, or continental shelf.
3. The very times when species are more likely to become
extinct than to evolve new forms—the times when the
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crust is subsiding, and terrestrial regions are being flooded
and plunged into dark depths—are the very times when
fossils are most likely to form; conversely, the times when
species are most likely to evolve—the times when the crust
is rising, and new terrestrial surfaces (as well as intertidal
and subtidal surfaces) are being formed—are the times
when fossils are least likely to form.
Therefore if a species, or whole group, evolved under
conditions when fossils were not forming, and were then fossilized,
they would appear in the fossil record as if they were
specially created. While positive evidence is trustworthy—we
can trust a fossil when we find it—negative evidence (the
absence of fossils) tells us nothing worthwhile. [This is one
of the assertions most vigorously debated regarding punctuated
equilibria.]
While this may explain why the geological record, on a
worldwide scale, does not preserve evidence of innumerable
transitional forms, it does not explain why innumerable transitional
forms are not preserved within any one formation. In
particular, some species seem to appear suddenly in the fossil
record, without predecessors leading up to it, even when there
has been no interruption of deposition. The appearance of a
species in the fossil record is much more likely to be the result
of immigration from a different region, where it had already
evolved, than of its sudden origin in that location. In some
cases a species seems to disappear, then reappear, then disappear,
then reappear; this is not because it re-evolved several
times, but because it vanished, then migrated back in from
another area, several times. Even within any one formation,
deposition may not have been uninterrupted; each formation
probably represents a whole series of switches between erosion
and deposition.
A more serious problem is what appears to be the
sudden appearance of a great variety of multicellular lifeforms
at the beginning of the Cambrian period (see Cambrian
explosion). My theory, if true, should explain the
origin of all these forms, gradually, from a common ancestor;
therefore the world must have swarmed with creatures
even before this time. There is even a good question
as to whether the Earth has existed long enough for the
diversity of forms seen in the Cambrian deposits to have
evolved, if William Thomson’s calculations are correct. [It
turned out that they were not; see Kelvin, Lord.] I can
give no satisfactory explanation for the absence of fossils
in Precambrian rocks. The presence of organic matter in
these deposits indicates that life existed throughout that
time, a fact also demonstrated by the fossil of Eozoon. [It
turned out that Eozoon was not a fossil; however, numerous
unicellular fossils, and a few multicellular ones, have
been found in Precambrian deposits since Darwin’s time.
Darwin was right that life, albeit primarily single-celled,
existed throughout that time.] The case must at present
remain inexplicable; and it may be truly urged as a valid
argument against the views here entertained. [The discovery
of the slightly more ancient Ediacaran organisms has
not helped to, even now, solve the problem of the relatively
rapid origin of multicellular life.]