Breaking the Cambrian barrier

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Answering Darwin’s big question

Trust Charles Darwin to be his own severest critic. Having expounded a revolutionary evolutionary theory of natural selection, he realized that the past gives birth to the present. Darwin knew about fossils, including the famous, three-section trilobites, that dated to the Cambrian period, now known to have begun about 540 million years ago.

Never one to duck logic, Darwin wrote:

“Consequently, if the theory be true, it is indisputable that, before the lowest Silurian or Cambrian stratum was deposited long periods elapsed … and that during these vast periods the world swarmed with living creatures, yet why we do not find rich fossiliferous deposits belonging to these assumed periods … I can give no satisfactory answer.”

Photo: Asaphiscus wheeleri, TheoricienQuantique
In Darwin’s time, trilobites were considered evidence for some of the earliest life. But Darwin was right – life had been around for “vast periods” before the trilobites.

Indeed, according to J. William Schopf, professor and director of the Center for the Study of Evolution and the Origin of Life at UCLA, what came before was totally mysterious when Darwin wrote “Origin of Species” in the 1850s. “Darwin knew about the Cambrian era, and the big extinctions after that were known, but he knew nothing about the earlier fossil record. This was the case for about 100 years.”

And then, starting in 1953, University of Wisconsin-Madison geologist Stanley Tyler noticed ring-like structures in rocks in Minnesota and Ontario’s Gunflint formation.

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Older and slightly big man standing next to tower-like rock with his left hand resting on it

Photo: Department of Geoscience, University of Wisconsin-Madison
Stanley Tyler had a penchant for old rocks–from Ontario’s Gunflint formation to Wisconsin’s Van Hise Rock, which he is standing next to here.

The rock — a fine-grained quartz relative called chert — was 1.9 billion years old – almost four times as old as the earliest Cambrian fossils.

Tyler, collaborating with Elso Barghorn at Harvard, recognized the circular structures as stromatolites, mushroom-shaped rocks formed by layers of microorganisms called cyanobacteria. In 1965, the two reported that stromatolites were the oldest fossils ever seen.1

I can see you now!

Why did it take so long for Precambrian life to be recognized? “They had assumed that it would be like younger life, there would be coral, snails and trilobites,” said Schopf, an expert on the oldest life. “The basic problem was that a wrong assumption had been made. Life in the Precambrian turned out to be substantively different in organization and size.”

By exploring the interior of rocks using an increasing array of scientific techniques, Schopf and a growing group of colleagues have found life as early as 3.5 billion years ago.

Not bad for a planet with an estimated age of 4.7 billion years.

Double-not-bad, considering the exceeding scarcity of truly ancient rocks, hidden through the constant tectonic churning of the crust. The oldest rocks yet located are 3.8 billion years old, but any fossils they contain have been distorted by severe heat and pressure.

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Shallow ocean bay with outcropping of hundreds of black rock mounds

Stromatolites provide some of the best proof of ancient life. These grow in Hamelin Pool Marine Nature Reserve, Shark Bay, Western Australia.
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Slab of gray rock with horizontal lines from top to bottom indicating ancient layers

This cross-section of an Early Archean stromatolite shows black layers of “cooked” organic material — remains of the ancient microorganisms that formed the stromatolite.

Still, Schopf said, four lines of evidence show the ancient roots of life on our planet: microfossils, molecular biomarkers, proportions of carbon isotopes and stromatolites. Stromatolites are layered rock formed by layers of microorganisms called cyanobacteria (formerly blue-green algae), which produce oxygen in sunlight.

While some of the fossilized microorganisms found in ancient rock apparently have gone extinct, the cyanobacteria closely resemble living organisms, Schopf told an audience at the University of Wisconsin-Madison on April 26. “Cyanobacteria do the same sort of photosynthesis as a blade of grass today. These are the guys that invented this process, probably 3-plus billion years ago.”

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Closeup of translucent bacteria that look like a string of beads

Photo: University of Wisconsin Plant Teaching Collection
These cyanobacteria, magnified 100 times, are a modern relative of the microorganisms that formed stromatolites billions of year ago.

As testimony to nature’s predilection for retaining stuff that works, other fossil microorganisms resemble modern counterparts that require oxygen, cannot tolerate oxygen, or use it when convenient. “We’ve found 12 to 15 major families of cyanobacteria, the same ones that are important today, the same ones that are seen throughout the geological record,” Schopf says.

Tyler did not live to see the publication of his 1965 article, but it revolutionized paleontology, and has been cited by scientists at least six times since 2010.

“Stanley Tyler was a hero for this world,” says Schopf. “As [microbiologist Louis] Pasteur said, chance favors a prepared mind. Here was an economic geologist [concerned with finding minerals and mines] … and yet he saw these scrubbly things, and thought, ‘I bet they are fossils,’ even though they were almost two billion years old. This is the guy who made the discovery.”

– David Tenenbaum

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Terry Devitt, editor; S.V. Medaris, designer/illustrator; Jenny Seifert, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive

Bibliography

  1. Microorganisms from the Gunflint Chert, Elso Barghorn and Stanley, Tyler, Science 5 February 1965:
    Vol. 147 no. 3658 pp. 563-575, DOI: 10.1126/science.147.3658.563
  2. Darwin’s dilemma
  3. Precambrian life
  4. History of life on Earth.
  5. More origins of life.
  6. NASA Astrobiology Institute.
  7. Stomatolites.
  8. The oldest fossils.
  9. Stromatolites then and now.
  10. Cyanobacteria fossil record.
  11. Stromatolite interactive gallery.
  12. Tyler’s discovery in Time Magazine.
  13. Life on Mars?