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07/25/2007
Surprising Shared Heritage
Recently our Star-Ledger had two articles pertaining to the
world’s tallest man, 56-year-old Mongolian herdsman Bao
Xishun, who stands at 7 feet 9 inches. Some time ago, Bao made
the news when he used his extra long arms to reach inside two
dolphins’ stomachs to pull out some pieces of plastic threatening
their lives. One Star-Ledger article concerned Bao’s recent
marriage to a 5-foot 6-inch 28-year-old woman. The second
article showed a picture of Bao meeting 19-year-old He
Pingping, who is claiming the title of the world’s shortest man at
an inch or so less than two and a half feet tall! The picture of the
two together was the ultimate in contrast.
An AP article dated July 11 on AOL News featured another very
long individual of a different species. This was a huge 550-
pound squid that washed ashore in Australia. It measured 26 feet
from the tip of its head to the ends of its tentacles. Our local
farmers’ market has a fishmonger who sells squid and I couldn’t
help thinking what a bonanza it would be if he stumbled upon
such a squid. However, the article said that the giant squid
would not be a tasty dish thanks to the large amount of ammonia
it contains to help buoy it up in the water. Actually, this squid
was only half as big as an 1100-pound, 33-foot giant squid netted
by New Zealand fishermen back in February.
In contrast to these giant squids, the starlet sea anemone is a
small tube-like sea creature measuring just a few centimeters in
length. This sea anemone joins the ranks of species such as us
humans having its DNA decoded. In the July 6 issue of Science,
Elizabeth Pennisi discusses another paper in the same issue by
Nicholas Putnam of the Department of Energy Joint Genome
Institute in California and a host of other authors from
institutions located from Hawaii to Bergen, Norway. (I can still
remember being in Bergen in 1970 and the taste of freshly caught
shrimp (or was it crayfish?) cooked on the docks.)
Why should we be interested in the DNA of the sea anemone? It
has to do with our roots. It turns out that all us “tissue-grade”
animals, known as eumetazoans, are descended from a common
ancestor that lived some 700 million years ago. I gather from the
two Science articles that the sponge is not a tissue-grade animal
but other animals are “eumetazoans”. Where do we fall in the
eumetazoan mix? Along with flies, worms, snails and a host of
other creatures, we humans are “bilaterians”. Comb jellies fall in
a separate category known as “ctenophores” while jellyfish,
hydra and our sea anemone fall in a class known as “cnidarians”.
Unfortunately, no fossils have been found of the common
ancestor that gave rise to all of us eumetazoan animals.
However, we apparently do know that the cnidarians branched
off our common ancestor’s line earlier in the game than we
bilaterians came into being. The cnidarian sea anemone is a
simple sort of sac-like animal with only a mouth, no anus,
tentacles and a net of nerves but no central nervous system as
such. The name cnidarian comes from the term for its stinging
cells known as “cnidocytes”.
What caught my attention, aside from the DNA results, which
we’ll get to shortly, were the interesting options the starlet
anemone has for reproducing. A picture in Pennisi’s article
shows the anemone releasing eggs into the water, which I
presume will get fertilized in the usual manner. However, also
shown is a picture of the alternative, which is to grow another
head at the opposite end from its original head! The anemone
then splits in the middle and voila! Two anemones.
Now for the genetic surprise. The DNA of the anemone more
closely resembles our DNA, the DNA of a vertebrate, than it
does the DNA of our bilaterian cousins the fruit fly or the wormy
nematode. Out of the anemone’s roughly 18,000 protein-coding
genes, almost 7800 are present in us bilaterians. Even large
numbers of the base sequences comprising DNA that aren’t parts
of genes, what used to be called junk DNA, are similar or the
same as in the DNA of vertebrates such as ourselves. In contrast,
well over a thousand of the genes found in us and in the anemone
have been lost by our nematode and fruit fly bilaterian cousins.
The surprising conclusion from these studies is that the anemone,
which certainly bears little resemblance to us, may tell us more
about the development and evolution of our own genetic heritage
than members of our own group of bilaterians. It now seems that
our common ancestor of over 700 million years ago had already
developed or evolved a large number of the genes and other
DNA code that we share with the cnidarians. Otherwise, how
could it be that we and the anemone share so much of our genetic
code after all these millions of years of going our separate ways?
Pennisi also cites work by John Finnerty and grad student James
Sullivan of Boston University to be published in the July issue of
Genome. They looked for genes in the anemone genome that
correspond to human genes involved in various diseases. Out of
286 human genes, they found 226 of them in the sea anemone!
Not only that but for some, such as one breast cancer gene, the
anemone gene was more like the human gene than the
corresponding genes found in fruit flies or nematodes.
Finally, shouldn’t we be glad that so far we humans haven’t
developed the alternative approach to reproduction of growing
another head and splitting in two?
Allen F. Bortrum
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