Fetus' Feet Show Fish, Reptile Vestiges - Or, Recapitulation lives!

[Colin McHenry <cmchenry@westserv.net.au>]

Found this on the Discovery web page. Love the way it's been written up. :-)

http://dsc.discovery.com/news/briefs/20060515/mammalfeet_arc.html (text
copied below)

Cheers
Col


Fetus' Feet Show Fish, Reptile Vestiges
By Jennifer Viegas, Discovery News


May 18, 2006 — The feet of human embryos taking shape in the womb reveal
links to prehistoric fish and reptiles, a new study finds.

Human feet may not look reptilian once babies emerge from the womb, but
during development the appendages appear similar to prehistoric fish and
reptiles. The finding supports the theory that mammalian feet evolved
from ancient mammal-like reptiles that, in turn, evolved from fish.

It also suggests that evolution -- whether that of a species over time
or the developmental course of a single organism -- follows distinct
patterns.

In this case, the evolution of mammalian feet from fish fins to
four-legged reptiles to four-limbed mammals to human feet appears to
roughly mirror what happens to a maturing human embryo.

"Undoubtedly there are clear parallels between the mammal-like reptilian
foot and the human foot," said Albert Isidro, an anthropologist at the
Autonomous University of Barcelona, Spain and lead author of the study,
which appeared in the journal The Foot.

Isidro and colleague Teresa Vazquez made the determination after
analyzing fossils of a number of mammal-like reptiles that lived from 75
to 360 million years ago. The scientists also studied fossils of
osteolepiform fish, which appear to be half fish and half reptilian.
These fish lived 400 million years ago and had lungs, nostrils and four
fins located where limbs would later be found in four-footed reptiles
and mammals.

In 33-day-old human embryos, the scientists observed "the outline of a
lower extremity in the form of a fin, similar to that seen in
osteolepiform fishes." As the embryo continued to develop, the
researchers focused their attention on two foot bones: the calcaneous,
or heel bone, and the talus, which sits between the heel and the lower leg.

At 54 days of gestation, these two bones sit next to each other as they
did within the reptile herbivore Bauria cynops, which lived around 260
million years ago. This ancient reptile had flat, crushing teeth and
mammalian features.

At eight and a half weeks of gestation, the researchers found the two
embryonic foot bones resemble those seen in the Diademodon vegetarian
dinosaur, which lived around 230 million years ago.

"We can tell that the embryo is half way between the reptiles and the
mammals (at this stage)," Isidro told Discovery News.

The two foot bones continue to develop until, at nine weeks, they
resemble that of placental mammals as they emerged 80 million years ago.

This development of feet in the human embryo mirrors how the foot
evolved over millions of years beginning with fish and ending with early
mammals, according to the scientists.

Supporting the fish/foot link was the discovery last month of a new
species, Tiktaalik roseae, which lived 375 million years ago. It had
fish fins and scales, but also limb parts found in four-legged animals.

"Tiktaalik blurs the boundary between fish and land-living animals both
in terms of its anatomy and its way of life," said Neil Shubin,
professor and chairman of organismal biology at the University of
Chicago and co-author of a related paper in the journal Nature.

H. Richard Lane, director of sedimentary geology and paleobiology at the
National Science Foundation, said, "These exciting discoveries are
providing fossil ‘Rosetta Stones’ for a deeper understanding of this
evolutionary milestone: fish to land-roaming tetrapods (four limbed
animals)."


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Colin McHenry
School of Environmental and Life Sciences (Geology)
University of Newcastle
Callaghan NSW 2308
Australia
Tel: +61 2 4921 5404
Fax: + 61 2 4921 6925

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