JOHN NOBLE WILFORD
Three Fossil Discoveries Clarify the Murky Origins of Life
from John Noble Wilford's "Three Fossil
Discoveries Clarify the Murky Origins of Life," New York Times, Oct. 31,1995,
C1-C7 (excerpted in Reilly's Worlds of History reader.
This article from the New York Times was written to inform readers about developments in science. What current developments does the author report? What is the significance of the date for the first multi-cellular life forms? What was the "Cambrian Explosion" of life forms?
Thinking Historically
After you read the article, note the importance mentioned: 4.6 billion years
ago, 3.8 billion years years ago, and 543 million years ago.
Create a time line by running a line across the bottom of a
notebook page and marking off ten equal sections. On the far left, label the
first notch 5 billion years ago, the next 4.5 billion years ago, and so on until
you reach 0.5 billion years ago. At the far right, the last notch, write
"today." Next, note on your time line the dates mentioned in the
article -to simplify matters, you might round them off to increments of 0.5
billion years.
Now recall what these dates mean. Think of each date as a
turning point in the history of life on Earth. Write a one-paragraph history of
life on Earth that includes these dates. Illustrate your history by drawing a
graph -using your time line as the basis -to show the increase in life forms
during the last five billion years. Could you argue that your paragraph and
graph constitute a "world history" ?
Last, note how often scientists and universities from various
parts of the world are mentioned in the article. Try to find these locations on
a world map. What does the participation of these different people tell you
about science today ?
Life, as far as scientists can tell, gained a foothold on
earth almost as rising above its tell, gained a foothold on earth almost as soon
as possible, then took an exceedingly long time rising above its simple origins,
and finally, 530 million years ago, erupted in a springtime of riotous
proliferation. In an astonishingly brief time, insects, earthworms, corals,
sponges, mollusks, and animals with rudimentary backbones -all the major body
plans of today -made their first appearance in what is known as the Cambrian
Explosion.
But there are many yawning gaps in this early history of
life, and so scientists welcomed reports last week of three discoveries. The
reports offered important refinements in the timing of three events that have
puzzled scientists trying to reconstruct the mysterious first steps in the
emergence of life. The events are the introduction of large multi-cellular
organisms, the existence of some flat jellyfish-like organisms as possible
predecessors of Cambrian life, and the emergence of chordates, the core group of
vertebrates that would eventually include humans.
Dr. Steven M. Stanley, a paleontologist at Johns Hopkins
University in Baltimore, said the findings were the latest manifestation of a
"really exciting and most important activity in the field, the development
of much better chronologies that constrain our views of what actually
happened."
In the beginning, 4.6 billion years ago, the planet was
covered with molten rock and bombarded steadily by swarms of meteorites. Not
until the surface cooled down about 4 billion years ago could there be life, the
first evidence for which are 3.8 billion-year-old fossils of a kind of
blue-green algae similar to pond scum. These were simple organisms with single
cells lacking nuclei. It was apparently another 2 billion years before more
complex cells with nuclei evolved. Until recently, little evidence existed for
multi-cellular organisms before a billion years ago.
Now, digging in sediments in northern China near Jixian,
Chinese geologists say they have gathered evidence suggesting a much earlier
emergence of more complex life than previously thought. They found more than 300
fossils of leaf-like multi-cellular plants that lived on the sea floor 1.7 billion
years ago. These were described as resembling longfengshanids, which lived 700
million years later and were assumed to be the earliest reliably dated
multi-cellular organisms.
The Chinese scientists acknowledged the discovery in Michigan
of an even older spaghetti-shaped organism, but suggested that it and other
early fossils " are not confidently interpreted " as multi-cellular
plants.
Writing in the current issue of the journal Science, Dr.
Zhu Shixing and Dr. Chen Huineng of the Chinese Academy of Geological Sciences
in Tianjin said the newly discovered fossils "imply that mega-scopic
multi-cellular organisms originated 1.7 billion years ago or earlier. " As
such, they added, the new data "have implications for the understanding of
the evolution and other related aspects of Precambrian life. "
Another discovery may solve the mystery of what has been
called a "broken link" in the poorly understood evolutionary chain
prior to the Cambrian Explosion.
For half a century, scientists have not known what to make of
creatures resembling jellyfish that were found in ancient sediments of the
Ediacara Hills of southern Australia and subsequently in fossil beds elsewhere
in the world. Were these plants or animals? Precursors of later life or a failed
experiment in biological innovation that came to a dead end? The problem was
determining exactly when they lived and if, as it once seemed, they died out
before the Cambrian Explosion and thus could not be directly ancestral to any of
the new life forms.
Applying more precise dating technologies to Ediacaran
fossils from the deserts of Namibia in southern Africa, geologists at the
Massachusetts Institute of Technology and Harvard University determined that the
youngest organisms had indeed survived into the early period of the Cambrian,
543 million years ago. The team, led by Dr. John Grotzinger of M.I. T .,
reported the results in Science.
"If Grotzinger and company are correct, that's excellent news," Dr.
Simon Conway Morris, a paleontologist at Cambridge University in England, was
quoted as saying in an accompanying article in the journal.
Scientists said the new evidence left open the possibility
that there was after all no broken link in the evolutionary chain and that
Ediacaran organisms could have played a role in the development of a multitude
of flora and fauna that characterize the Cambrian period and are the
predecessors of life on earth today.
Dr. Samuel Bowring, an M.I. T. geologist on the research
team, said, "What this shows is that evolution likely proceeded smoothly as
opposed to having a period of evolution followed by an extinction, which would
open ecological niches allowing other life forms to develop."
The dating was done on grains of the mineral zircon found in
trace amounts in volcanic ash. By analyzing the decay rates of uranium into
lead, the geologists obtained dates for the fossil-bearing sediments that they
say are accurate to within plus or minus one million years, a refinement
previously unattainable on samples that old.
"Five to ten years ago, being able to date something to
within five million years was a major achievement," Dr. Bowring said.
"The more precisely we can resolve time, the more sophisticated the
evolutionary questions we can address. "
Dr. Stanley of Johns Hopkins suggested that the Ediacaran
fossil record might have been deceptive. Evidence for these soft-bodied
organisms from the sea floors was found in such profusion in sediments just
before the Cambrian period because nothing was scavenging on them at that time.
Then the fossils seemed to disappear. Was this the sign of a true extinction, or
merely an absence of fossil remains of that particular life? With the greater
diversity of life in the Cambrian, he said, there could have been many
scavengers munching on the Ediacara organisms before they had a chance to become
fossils.
The most abundant remains of animal life originating in the
Cambrian period are found in the Burgess Shale, fossil beds in the Canadian
Rockies that have been the main source of knowledge about this time. But for the
last decade, paleontologists have been mining an important new source at
Chengjiang in the Chinese province of Yunnan, which was the site of the third
discovery reported last week.
An international team of scientists described finding what
may be the earliest known representative of the Chordate, the branch of the
animal kingdom that includes vertebrates and two lesser known allied forms of
life. Previous generations of scientists had thought that chordates evolved in
the later Ordovician geological period. Later evolution, they liked to think,
could imply advanced and special status to the branch of life leading to humans.
In a report in the journal Nature, Dr. Lars Ramskold,
a paleontologist at the University of Uppsala in Sweden, and colleagues said
they had identified 525-million-year-old fossils of a strange, fishlike
creature, which they have named Yunnanozoon lividum. One of the characteristics
linking the specimen to chordates is its notochord, the precursor of a spinal
column.
The researchers said Yunnanozoon appeared to belong to the
division of chordates known as cephalochordates, which are closely related to
backboned animals, including humans, but not of them. Current representatives of
the group include amphioxus, a shy marine creature.
The identification of the new specimen as a chordate will be
controversial, scientists said. The only other possible chordate from the
Cambrian period, the Pikaia from the Burgess Shale, has not been described in a
sufficiently detailed report and so has yet to be accorded full scientific
standing. But the discoverers of the Chengjiang fossil said that the presence of
one division of chordates in the Cambrian period indicated that the entire
branch probably existed then as well.
In a commentary accompanying the chordate report, Dr. Stephen
Jay Could, a Harvard paleontologist and evolutionary
biologist, said the "unambiguously identified chordate from the still
earlier Chengjiang fauna now seals the fate" of previous efforts to assert
the specialness of human ancestry by separating it from the herd of new Cambrian
animal forms.
"So much for chordate uniqueness marked by slightly later evolution," he wrote." As for our place in the history of life, we are of it, not it."