Re: Sharks, Humans Share Genetic Kinship


"Perplexed in Peoria" <jimmenegay@xxxxxxxxxxxxx> wrote in message
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"Peter Pan" <peterpan55555@xxxxxxxxx> wrote in message
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http://dsc.discovery.com/news/2007/05/21/sharkancestry_ani.html?category=animals&guid=20070521103030&dcitc=w19-502-ak-0000

"Four hundred and fifty million years ago, sharks and humans shared a
common ancestor, making sharks our distant cousins. According to
recent research, this kinship is evident in our DNA, since at least
one shark species possesses several genes that are nearly identical to
those in humans.

The elephant shark's genome is so similar to ours that we wind up
having more in common with it - genetically speaking - than with other
species, such as teleost (bony skeleton) fishes, which are nearer to
us on the evolutionary tree.

"This was a surprising finding, since teleost fish and humans are more
closely related than the elephant shark is to humans," lead author
Byrappa Venkatesh told Discovery News.

Venkatesh, principal investigator at the Institute of Molecular and
Cell Biology in Singapore, and his team determined that sets of genes
on chromosomes, as well as actual genetic sequences, are "highly
similar in the elephant shark and human genomes."

The researchers not only analyzed the elephant shark genome, but also
the genes for other animals including pufferfishes, chickens, mice and
dogs.

Their findings were recently published in the journal PloS Biology.

The researchers identified 154 genes in humans that have comparable
matches in mice, dogs and elephant sharks. The similarities between
people, mice and dogs were not unexpected, given that they are all
mammals. Sharks, however, are cartilaginous fish that seem to bear
little physical resemblance to mammals.

Upon closer examination, sharks and humans do share certain
physiological and biochemical processes.

http://dsc.discovery.com/news/2007/05/21/sharkancestry_ani_02.html?category=animals&guid=20070521103030

"The researchers also found that shark and human immune systems are
very similar, since sharks have all four types white blood cells found
in mammals.

Sean Van Sommeran, executive director of the Pelagic Shark Research
Foundation in Santa Cruz, Calif., told Discovery News that he was not
entirely surprised to learn about the shark-human links.

"The field of genetics is a Pandora's box," Van Sommeran said.

He added, "Sharks copulate like mammals and females give birth to live
young, so sharks do have features in common with mammals. It makes
sense that these would show up in the genome."

Venkatesh said future studies on the elephant shark genome, which is
relatively small and easy to study, could reveal information about
human genes, such as how the immune system develops. Since sharks are
the oldest living jawed creatures with a backbone, studies on them may
even uncover how humans and other mammals evolved.


This is a kick in the teeth for those who believe natural selection and
drift are sufficient explanations for the course of evolution.

We get kicked in the teeth by ignorant science journalists all the time.

But, in this case, I don't see anything in the article that hurts my
teeth. What, exactly, in this story do you see that creates problems
for the idea that NS and drift are sufficient? And 'sufficient' for what,
exactly?

Let me draw a tree [fixed width font]:

|----------------Sharks
|
----------| |--------------Teleost Fish
|----------|
|-----Quadrupeds

Now, as I understand it, the story says that there are several ways in
which sharks and quadrupeds are alike, to the exclusion of the teleosts.
Not exactly what you would expect from the tree, but not incompatible
with the tree either. And the explanation for this mild anomaly is well
known. There was an extra whole-genome-duplication on the teleost branch
which made the teleosts quite different from their first cousins, the
quadrupeds, and from their second cousins, the sharks. It is true that
NS and drift are not, by themselves, a sufficient explanation for why this
whole-genome-duplication occurred in only one branch, nor why it occurred
in that particular branch. But no one ever suggested that it was
sufficient.
And, if anyone did, then they deserved to get kicked in the teeth.

Let me draw a couple more trees to show that this kind of thing (tree
'anomalies', not genome duplications) is very common and well known:

|----------------Gorilla (48 chromosomes)
|
----------| |--------------Man (46 chromosomes)
|----------|
|-----Chimp (48 chromosomes)
and

|----------------Turtles (cold blooded, scales)
|
----------| |--------------Birds (warm-blooded, feathers)
|----------|
|-----Crocodiles (cold blooded, scales)

In each case, all it means is that some particular step in evolution took
place on the middle branch only. No big mystery.

Nice explanation, and I am somewhat averse at taking it further since mel
turner in a post above has pretty much portrayed the article as idiotic,
nevertheless I find the length of time troubling. Four hundred and fifty
million years ago the ancestors of shark and man split. One evolved to
survive in cold salt water, the other a hot terrestrial environment. The
steady, ongoing appearance of mutations should have produced a more thorough
divergence at the molecular level.

Your two trees involve qualitatively different cases. Turtles and crocodiles
have similar niches in comparison of either to birds. Likewise the chimp and
gorilla have closer niches than either does to man. The shark, however, has
a niche closer to teleosts than to man.

Despite the possibility of the shark maintaining certain molecular
similarities to humans which the teleosts lost, the opposite result is to be
expected.


.

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