NetNews Usenet Archive 1992 #27/NN_1992_27.iso/spool/talk/origins/14585

Text File | 1992-11-20 | 9.7 KB | 191 lines

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From: [e-mail] (Loren I. Petrich)
Subject: Re: Bad design and vestigial organs
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Date: Sat, 21 Nov 1992 01:34:11 GMT
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    In my article on vestigial features, I had promised to omit
the animal kingdom, in the expectation that others would have
superabundant animal-kingdom examples. That expectation only partially
fulfilled, I will now give some animal-kingdom examples. I hope it is
good FAQ material :-)


    The wings of flightless birds. For most flightless birds, the
wings are non-functional, aside from possible display functions. The
only major exceptions are diving birds, like penguins, whose "wings"
serve as control surfaces. In some cases, the wings are _very_ small,
as for kiwis. The effect is to reduce the number of usable limbs from
4 to 2, which can hardly be called an improvement.

    Bird-teeth genes. All the living birds, and all the known
Cenozoic fossil birds, are toothless. Most Mesozoic birds and
dinosaurs possessed teeth (any toothless Mesozoic birds?). A recent
experiment in growing chicken-embryo jaw tissue next to some mouse/rat
jaw tissue in a mouse's eye revealed that teeth formed. And the teeth
did not look like any rodent teeth, but were peg-shaped with a conical
top, just like the fossil bird teeth. The ability to grow teeth was
thus preserved for over 65 million years, perhaps as a side effect of
certain growth-control genes specifying more essential things.

    Extra toes of ungulates. Various hoofed mammals typically have
toe bones in addition to those that bear the hooves. This is readily
evident on the feet of artiodactyls (cows, deer, pigs, etc.). For
equids, two splints are sometimes present alongside the main toe bone.
Also, domestic horses are sometimes born with three-toed feet.
Relatively recent fossil equids, however, often had three-toed feet,
indicating that the one-toed feet of the extant equids is a
development of the last couple million years, but that the animals
still have the ability to produce three toes per foot.

    Solid-color equids having genes for making stripes. The living
equids are the domestic horse, its wild progenitors, the donkeys, and
the zebras and quaggas. Matings of different breeds of solid-color
equids (horses and donkeys) sometimes produce offspring with
zebra-like stripes. It is as if the genes for making stripes, which
are expressed in zebras, are switched off in the solid-color equids,
only to re-emerge in certain circumstances.

    Flies growing legs instead of antennae on their heads, and
mosquitoes with legs for mouthparts. These "homeotic mutations"
suggest that these appendages were originally legs, but that they were
specialized to different functions. Removing or disabling genetic
instructions which roughly translate into "A limb on this segment is
to become an antenna" and "a limb on this segment is to become a
mouthpart" leaves the limb following a default instruction that goes
something like "a limb on this segment is to become a leg" (it's not
even _that_ simple, because insect legs on different segments are
often specialized differently). There is another mutation that causes
fly larvae to start growing legs on the abdominal segments; this
mutation is lethal, but if it was not, then an adult fly would emerge
from the pupa with lots of extra legs down its body. The results of
these limb-growth-control mutations are consistent with the hypothesis
that the original arthropod had essentially identical, unspecialized
limbs, which were specialized to different functions, or even
suppressed, among its descendants.  These limbs would have been
specified in cookie-cutter fashion, and the various specializations
and suppressions would have resulted from later add-ons to the growth
instructions. Interestingly, trilobites and the Burgess Shale
arthropods show relatively little evidence of limb
specialization/suppression, so the earliest fossils are consistent
with the overlaid cookie-cutter hypothesis.

    Crab tails. Under their broad, flattened bodies can be found
small tails.  These are clearly a leftover from when their ancestors
had long, thin bodies, as lobsters still do.

    Ancestral wing configurations reappearing. Flies sometimes
grow a second pair of wings instead of halteres (balancing organs);
most other living insects have two pairs of wings. Cockroaches
sometimes grow a third pair of wings, like some fossil insects.

    Fetal teeth missing from adults. Baleen whale fetuses have
teeth and fetal calves have upper front teeth; adult (and probably
newborn) baleen whales are toothless (the baleen is not teeth), and
cows lack upper front teeth. These teeth never erupt and are resorbed
as the fetus grows.

    Snakes with vestigial limbs. Boa constrictors have small
vestigial hind legs; these may aid in copulating. However, most other
species of snakes lack this feature, and seem to do fine without them.

    Cetacean hipbones. Some whales have hipbones deep inside their
bodies, attached to no limbs. One possible purpose is to serve as an
attachment point for muscles that move the penis, however.

    Mammal tails, at least in many cases. These are much reduced
from the reptilian ancestral form, and when they serve a function, it
is usually for whisking away flies (as for horses) or for signaling
(consider dogs wagging their tails). New World monkeys, however, use
them as an extra limb, and kangaroos have big tails for balancing, so
mammal tails sometimes do have important new functions, however. There
are some with very tiny tails, like elephants, and some which lack
them, such as bears and apes/humans.  The ancestral ape was probably
capable of brachiating (moving around in trees suspended from tree
limbs that one is holding), which gibbons and siamangs still do today.
This would have made a tail a nuisance, thus leading to its
suppression (the same thing may have happened to the ancestor of the
frogs and toads). The disappearance (or only near-disappearance?) of
bear tails is less easily explainable, however. But even there,
evidence of tails is sometimes present, as in human embryos having
tails for awhile. A side effect of a brachiating ancestry may be our
ability to point our arms straight upward (in the direction of the
head), an ability not as critical for our species as it is for gibbons
and siamangs.

    Flounder eyes. On sea floors, there live these fish that lie
on their sides. They have two eyes -- on one side of their heads. But
they start off life with eyes on both sides of their heads, and one
eye moves to the other side. Why two eyes instead of one? And why
originally on both sides of the head?

    Original embryonic eye positions. In human and dog embryos, as
in most other vertebrate embryos, the eyes are originally on the sides
of the head. However, the eyes move forward as human and dog embryos
grow, to make possible binocular vision. One human birth defect is for
this process to be incomplete, making the eyes too far apart. Among
the vast majority of the animals with backbones, the eyes are at the
sides of the head; the main exceptions I know of are the bats, the
primates, the carnivores, the owls, and possibly some of the more
cerebrally endowed small carnivorous dinosaurs. In their family trees,
they are surrounded with eyes-on-the-side animals, suggesting that
binocular vision evolved several times.

    Giraffe neck lengths. Baby giraffes start out with necks whose
relative length is similar to those of other ungulates; it is as they
grow that they acquire the relatively long necks that the species is
noted for.

    Human toes. Our feet have toes, one of which is big and
slightly separated from the others. For walking, there is no special
need of having a split front end of the foot; it should not be
surprising that the toes are small. But they are there, and in most
primate species they are much more prominent. In some species at
least, the big toe points outward, just like a thumb. Interestingly,
in some early hominid species, the toe bones were relatively longer
than in our species.

    Wisdom teeth. Our jaws are a bit small for these late-erupting
teeth; some people have them, while others do not.

    Outsized hind legs of some four-legged dinosaurs.
_Stegosaurus_, especially, had hind legs much bigger than its front
legs. This is probably a byproduct of being descended from a
two-legged ancestor that went back to walking on all fours. Many of
the dinosaurs walked on their hind limbs only, with the front limbs
remining at various levels of development. In _Tyrannosaurus_, they
are _very_ small, though still there, which has led to the suggestion
that they are vestigial. The earliest dinosaurs known, like
_Herrerasaurus_, were like this. Transitional cases? Possibly!
_Iguanodon_ or some other such dinosaur apparently walked on two legs
when juvenile, and on all fours when adult (and a lot heavier).


    [My memory runs out at this point...]


    Good sources for some of this material: Charles Darwin's
_Origin of Species_ and Stephen Jay Gould's essays, notably _Hen's
Teeth and Horse's Toes_. In addition, studies of embryonic development
often reveal an abundance of vestigial features, some examples of
which are given here.


    On the molecular level again....

    An abundace of "pseudogenes" have been discovered, which are
not prefaced with a "start" codon, but which have a resemblance to
known genes that is too improbable to be coincidence. These are most
likely the results of gene duplications and mutations that turned the
"start" codon into something else. Thus the DNA-to-RNA transcription
system does not "know" that here is a gene to be expressed.

/Loren