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Unmuddying the Cambrian waters

March, 2016
By guest author Stephanie Keep, Editor of Reports of the National Center for Science Education, and science curriculum consultant for Keep Learning, LLC

recreated scene of Cambrian species

Painting by D.W. Miller

Any scholar of history knows that the further back in time you go seeking answers, the more sparse and unreliable the clues. Paleontology is the study of the history of life, and its clues — for the most part — take the form of fossils. The fossil record is notoriously incomplete, but in general, we have more direct evidence of life in the recent past than we do of life in the most distant past. It should therefore not be surprising that one of the most studied events in evolution, the Cambrian "explosion" (which started about 542 million years ago and lasted — unlike most explosions — over 10 million years), is also one of the most enigmatic and passionately debated. Can new research bring life of the Cambrian into focus?

Where's the evolution?

How old is life on Earth? The first records of life are indirect: researchers have tried to detect the chemical signature of life in graphite deposits and zircon crystals over 3.8 billion years old. The oldest true fossils, mats of unicellular cyanobacteria called stromatolites, date back to around 3.45 billion years. Fast-forward almost three billion years to the Ediacaran period, about 600 million years ago, and the first undisputed multicellular organisms are found. Known collectively as the Ediacaran biota, these organisms had fairly simple anatomies and lifestyles. Animals described by one journalist as "thin, quilted pillows" grazed on mats of microbial life while other, stationary, forms passively collected nutrients from the water. There were no predators and no evidence of modern anatomical features such as appendages or eyes. But that all changed around 540 million years ago.

In what is commonly called "the Cambrian explosion," a variety of animal forms burst onto the scene in a geological blink of an eye. (But remember, geological blinks take tens of millions of years!) By the time of the iconic Burgess Shale, in the middle Cambrian about 508 million years ago, animals recognizable as worms, arthropods, and even chordates are found. In fact, nearly every modern animal phylum can trace its origins to animals like those represented in this Cambrian assemblage. The conditions under which the Burgess Shale formed resulted in remarkable preservation of these middle Cambrian creatures, from which we are able to glean details of even their soft tissues. While Ediacaran forms were relatively featureless as far as the fossil record reveals, Cambrian life clearly boasted compound eyes, gills, jointed appendages, and jaws with teeth.

What spurred this burst of rapid evolutionary change? For some time now, the prevailing hypothesis has been that the Cambrian explosion was somehow tied to an increase in available oxygen. All organisms, from the smallest bacteria to the largest sequoia tree, have to break down food for energy. Food is any source of matter and energy — food can be carbon, sulfur, or iron absorbed by bacteria, or complex carbohydrates stored in the roots of plants, or the peanut-butter-and-jelly sandwich you had for lunch. There are two ways food can be broken down — without oxygen (anaerobic respiration), and with oxygen (aerobic respiration). Aerobic respiration is much more efficient than anaerobic respiration, releasing far more energy per unit of food. 

But aerobic respiration is only possible when free oxygen is available, and for billions of years, it wasn't; any oxygen produced by photosynthetic bacteria was absorbed by ferrous (iron-containing) rocks on land and in the sea. Could a sudden increase in available oxygen have made it possible for energy-demanding structures, such as muscles and a complex nervous system, to evolve?

To test this idea, scientists set out to analyze oceanic oxygen levels around the time of the Cambrian explosion. This is not as simple as taking direct measurements, however. There are no pockets of Cambrian ocean around to directly analyze, so proxy measurements are necessary. In this case, the amounts of certain metals incorporated into ancient rocks can be used to estimate how much dissolved oxygen was present when the rocks formed. Initial results suggested that oxygen levels reached modern levels just before the Cambrian explosion. This conclusion supported the hypothesis that an increase in available oxygen enabled the burst of complex life found in the Cambrian.

However, in 2015, a new study led by paleoecologist Erik Sperling of Stanford University came to a very different conclusion. Sperling was also looking at the metals contained in ancient seafloor rock, but unlike the earlier researchers, he did not detect anything close to modern oxygen concentrations around the time of the Cambrian explosion. Also in 2015, an international team from China and Denmark concluded that there was already enough oxygen available to support Ediacaran animals almost 100 million years before they show up in the fossil record. Just because there is enough oxygen for an evolutionary leap to occur is no guarantee that it will.

Taken together, these results muddy the Cambrian waters. Was there an increase in oxygen availability around 540 million years ago, or not? And even if there was, was that enough to spark the Cambrian explosion, or was something else needed?

Sperling turned to modern ecosystems for answers. He found plenty of examples of complex life in the modern ocean living in anoxic areas, that is areas with low oxygen concentrations — as low as 0.5% of average. With a bit more oxygen, between 0.5 and 3% of average, animals are more abundant, though there is generally a lack of predation. But between 3–10% there are complex food webs that include predatory relationships. Sperling reasoned that if this 3% threshold were crossed around 540 million years ago, predation would have been possible, which would have then started a cascade of rapid evolutionary change.

Once predatory animals evolved, an arms race between predator and prey would have commenced. Suddenly, strong selection pressures would favor the evolution of adaptations, from teeth and complex eyes to hard exoskeletons, burrowing behavior, and complex musculo-nervous systems. The Ediacaran forms, with their soft bodies and passive lifestyles, would have been easy prey and quickly driven to extinction. The Cambrian fauna, meanwhile, would have begun to move into new habitats, becoming burrowers or moving up in the water column, exploiting the resources they found there.

Sperling’s hypothesis, that a modest increase in oxygen levels enabled the evolution of predacious lifestyles, which in turn sparked an evolutionary arms race, is intriguing — but questions remain and research continues. In just the last few months, studies have been published examining how magnetic field fluctuations and a phenomenon known as "true polar wander" could have affected early animal evolution. Just as the discovery of a few fragile ancient clay tablets can dramatically add to our understanding of early human civilization, data from these studies contribute to our understanding of this murky period of evolution. The good news: the picture can only get clearer.


Read more about it

Primary literature:

  • Sperling, E.A., Wolock, C.J., Morgan, A.S., Gill, B.C., Kunzmann, M., Halverson, G.P., Macdonald, F.A., Knoll, A.H., Johnson, D.T. (2015). Statistical analysis of iron geochemical data suggests limited late Proterozoic oxygenation. Nature doi: 10.1038/nature14589
News articles:

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Discussion and extension questions

  1. In your own words, describe what scientists mean when they talk about the Cambrian explosion.
  2. If life has existed on Earth for 3.7 billion years, and multicellular life didn't appear until 600 million years ago, what proportion of life’s history is represented solely by unicellular life?
  3. What is the process by which new traits first arise?
  4. Explain how natural selection leads to the spread of adaptations through a population.
  5. Advanced: What is an evolutionary arms race? Use the Internet or other resources to find an example to illustrate your definition.
  6. Advanced: How do scientists estimate that the first life on Earth appeared 3.7 billion years ago if they don't have fossils older than 3.45 billion years? Use the Internet or other resources to research methods involving the detection of life’s "chemical signatures."
Related lessons and teaching resources

References

  • Zhang, S., Wang, X., Wang, H., Bjerrum, C.J., Hammarlund, E.U., Costa, M.M., Connelly, J.N., Zhang, B, Su, J., Candield, D.E. (2016). Sufficient oxygen for animal respiration 1,400 million years ago. PNAS 113(7): p. 1731–1736.
  • Mitchell, R.N., Raub, T.D., Silva, S.C., Kirschvink, J.L. (2015). Was the Cambrian explosion both an effect and an artifact of true polar wander? American Journal of Science 315: p. 945–957.