Earth,Animals,Dinosaurs & Fossils When life exploded

Earth,Animals,Dinosaurs & Fossils

When life explodedScientists probe what happened 540 million years ago to trigger the biggestemergence ever of animal species

By Beth Geiger 8:30am, November 13, 2014

In this artist’s conception of an ocean scene during the Cambrian Period, the fearsome predator at upper left,called Anomalocaris canadensis — or “unusual Canadian shrimp” — chases three trilobites.

Courtesy of Ken Doud

Flash back 540 million years. It’s the dawn of a new era, and something very, verybig is about to happen to life on Earth. Over the next 10 million years — a blink ofan eye in our planet’s long history — dozens of new forms of animals will suddenlyemerge in the oceans.

This burst of life marked the start of a period in Earth’s history called theCambrian. It was so dramatic and so fast that scientists call it the Cambrian

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An artist’s conception of an Opabinia, one of the trulystrange Cambrian creatures. This thumb-sized creaturehad five mushroom-shaped eyes and a miniature trunklike an elephant’s, with a handy gripper at its end.Nobu Tamura (http://spinops.blogspot.com)/WikimediaCommons

Explosion.

“The Cambrian Explosion was the biggest diversification in animals in the historyof life,” says David Harper. This paleontologist works at Durham University inEngland.

Before the Cambrian, Harper says, marine animals had been divided among just ahandful of groups, or phyla (FY-luh). Life forms had been relatively simple, such asbacteria and some worms. Now, the oceans suddenly teemed with creatures.Some had hard shells. Others had nerve chords. Many could wiggle, burrow orswim. They had developed different organs for different functions, including eyes.

Ninety percent of the animal phyla that exist today appeared in that short windowof time. The animals in each phylum generally share a similar body type or plan.One of the new groups was the chordates — animals with backbones. That grouptoday includes all fish, reptiles, birds, amphibians and mammals. Another was thearthropods, which now includes insects, arachnids (A-RAK-nidz) and crustaceans(Krus-TAY-shuns). Animals with radial symmetry, such as the ancestors of starfishand sea urchins, also emerged at this time. These animals are known asechinoderms (Ee-KI-no-derms).

Never again have so many newtypes of animals emerged in such ashort period of time. (Although sincethen several mass extinctions havewiped out nearly as many speciesjust as quickly.)

No one really knows what actuallytriggered this burst of life, whichwas unique in our planet’s history.But recently, studies have turned upnew clues to how changes in thegeology, chemistry and atmosphereof Earth influenced the CambrianExplosion — and how this rapidgrowth in the diversity of life, inturn, affected the planet.

Cambrian conundrum

An amazing glimpse into the Cambrian Explosion comes from a rock formationfound in the Canadian Rockies of British Columbia. This Burgess Shale preservescountless fossils of Cambrian creatures. The organisms became fossilized after


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Charles Doolittle Walcott (left) discovered the BurgessShale in the Canadian Rockies in 1909. Here, he searchesthrough rocks for fossils with his son Sidney (center) anddaughter Helen (right), circa 1913.Smithsonian Institution/Wikimedia Commons

they were trapped in muddy marine sediments that later turned to stone. Thefossils record fine details of these creatures. And not just of hard body parts suchas bones and shells, but also of muscles, guts and other soft parts.

Some Burgess Shale creatures seema little familiar. Others are just plainstrange. Take Opabinia, for example.This thumb-sized creature had fivemushroom-shaped eyes and aminiature trunk like an elephant’s,with a handy gripper at its end.Opabinia kept odd company, too.One critter from its period, forinstance, was almost all legs andspikes — like a walking, armoredslug. It was so weird that biologistshave named it Hallucigenia — as ifonly a hallucination could explainsuch a creature.

The Burgess fossils reveal a fascinating world. Yet the Cambrian Explosion was farmore than just a lot of bizarre new animals. “The big thing about the CambrianExplosion is that it was the first animal-based community,” Harper told ScienceNews for Students. “Before that, animals were there, but they weren’t verydiverse or common.”

Oxygen on the rise

Earlier, during a very long stretch of time known as the Precambrian, life wassimple. Bacteria dominated, quietly clumping into slimy mats. Then, startingaround 3.5 billion years ago, some of those bacteria began producing oxygen as abyproduct of photosynthesis. Until then, there had been little or no oxygen, whichcomplex animals need.

Around 2.3 billion years ago, oxygen levels in the atmosphere and oceanssuddenly rose. Iron began to react with that oxygen in a process called oxidation.You might think of it as rusting. The rusting that accompanied that spike in theair’s oxygen levels is permanently recorded in rock layers. These include somethat are called banded iron formations.

What happened to oxygen levels over the next 1.8 billion years, leading up to theCambrian Explosion, isn’t clear. Scientists want to know: Did oxygen levels risegradually over this period? Or did oxygen levels skyrocket, providing a quicktrigger for life’s great blossoming?


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Halite is a mineral that forms when seawater evaporates.Researchers discovered seawater droplets inside 830million-year-old halite crystals that allowed them toestimate oxygen levels in the Precambrian ocean. Suchmeasurements allow experts to see how a steadyincrease in oxygen helped set the stage for the CambrianExplosion.iropa/istockphoto

Time capsules

Imagine being able to test drops ofactual Precambrian seawater to findout just how much oxygen theoceans contained at a givenmoment in the distant past. That’swhat Natalie Spear has done. Lastyear, this geologist and her team atPennsylvania State University inState College discovered ancientseawater droplets in Australia. Theywere trapped in 830 million-year-oldsalt crystals. When saltwaterevaporates, it leaves behind thismineral, called halite.

“The seawater was trapped as thesalt crystals grew,” Spear explains.Finding these crystals was “prettyremarkable,” she adds. “It’s difficult to find salt that old that is well-preserved.”The halite samples contained seawater nearly 300 million years older than anyever analyzed!

Liquid trapped inside a crystal is called a fluid inclusion. “Fluid inclusions are littletime capsules,” explains Linda Kah. A geologist at the University of Tennessee, inKnoxville, she studies rocks from the Precambrian to determine the chemicalconditions that existed when they formed.

Oxygen itself disappears when seawater evaporates. So Spear analyzed theancient seawater for a form of sulfur. It’s called sulfate. Measuring sulfate gaveher team a way to estimate how much oxygen the ancient seawater once held.

That may sound odd. But it works because oxygen in the air weathers, or erodes,rocks on land that contain iron and sulfur. “Even a little oxygen in the atmospherewill weather rocks,” says Kah.

Rain then washes the eroded bits of rusted rock into the ocean, sulfur and all.There, oxygen reacts with sulfur to form sulfate. The more oxygen there is toweather rocks, the more sulfate that ends up in the ocean.

Spear’s team found sulfate levels 830 million years ago were just 10 percent ashigh as they are today. That level would be consistent with an atmospherecontaining about 2 percent oxygen. For comparison, oxygen today comprisesabout 21 percent of the air.


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A visitor to Canada’s Burgess Shale formation holds upseveral fossilized trilobites. The marine animals wereabundant during the Cambrian, when life exploded indiversity.Courtesy of Mary Caperton Morton/The Blonde Coyote(www.theblondecoyote.com).

Scientific studies of other seawaterinclusions offer clues to sulfatelevels — and oxygen — at points intime even closer to the Cambrian.They show that by the end of thePrecambrian, about 540 millionyears ago, ocean sulfate levels hadrisen to between 50 percent and 80percent of today’s levels. Oxygenlevels would have risen by a similaramount, says Spear at Penn State.Her team described its new findingsin the February 2014 Geology.

Another recent study also showedthat low oxygen levels played a rolein delaying the great diversificationof life. This study measured the riseof oxygen by analyzing trace metalsin layers of ancient rock. Aninternational team of researchers,led by Noah Planavsky of YaleUniversity in New Haven, Conn.,published its findings in the October31, 2014, issue of Science.

Together, these studies providesnapshots of how much richer inoxygen the world’s oceans grew inthe buildup to the CambrianExplosion.

Habitat helpers

The steady increase in oxygen levels over hundreds of millions of years certainlycould have set the stage for the upcoming explosion in animal life.

Yet oxygen wasn’t the only thing that changed during the last 300 million years ofthe Precambrian. Major ice ages came and went. Along with cold weather, theseice ages made sea levels yo-yo up and down. Also, Earth’s great tectonicplates were on the move. They changed the size and position of the continents —and in turn the circulation patterns in the oceans and atmosphere. All of thesechanges to Earth would have affected the life upon it.


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This fossil from Utah reveals a Cambrian worm. Theburrowing of these worms helped mix oxygen intosediments at the bottom of shallow seas. More oxygen, inturn, made the sediments even more habitable to othercreatures. Courtesy of the Virtual Fossil Museum,www.fossilmuseum.net

The simpler life forms that hademerged before the CambrianExplosion helped set the stage too,says Don Canfield. He’s a geologistat the University of SouthernDenmark, in Odense. There,Canfield studies the chemistry ofancient oceans.

Marine animals, such as worms,already were burrowing under theoceans, for example — 10 millionyears before the Cambrian began,he notes. And that churning likelychanged the chemistry of thesediments blanketing the shallowseafloor, he told Science News forStudents. Crucially, the action of theworms mixed in more oxygen,making the sediment even betterhabitat for animals.

Cascade of events

Harper, the Durham University paleontologist, has considered all of the factorsthat may have contributed to the Cambrian Explosion. He’s read about whatothers have found — Precambrian fossils, new estimates of ancient oxygen levels,shifting continents and evolving climates. He’s also probed rocks and fossils forclues.

Last year, working with M. Paul Smith at England’s Oxford University, Harperreached a conclusion about the cause of the Cambrian Explosion: “It was acascade of events, not one thing.” Life, he adds, got a lucky break as a series ofchanges built up. Harper and Smith published their assessment in the September20, 2013, Science.

Harper calls these changes “feedback loops.” What he means is that as onecondition changed, another changed in response. And this caused the firstcondition to change some more. The process sped up adaptation and change.

Harper says predator-prey relations are a great example. As predators evolve,their prey must evolve too if it hopes to avoid being eaten. Prey may developthicker shells or start burrowing. “Then,” he says, ”the predators respond bychanging too.” The results ripple throughout the ecosystem, Harper says. For


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The Burgess Shale has revealed all sorts of odd animals,such as the spine-covered Wiwaxia corrugate. Therelationship between predators and their prey probablyhelped drive the evolution of so many new organismsduring the Cambrian, as species competed to eat — andnot be eaten.Martin R. Smith/Wikimedia Commons

example, Hallucigenia most likelydeveloped those nasty spines todiscourage predators.

Meanwhile, geological changesplayed a role, too. As ice melted,sea levels rose. Those rising seasflooded shorelines. This, in turn,created lots more shallowunderwater habitat for animals,such as those burrowing worms thatCanfield described. More burrowingwould have moved more oxygeninto sediments. And more oxygen inturn would have spurred moreburrowing.

Rising seas also eroded moreshoreline, says Harper. That, in turn,washed more minerals into theoceans. The elements in themincluded, of course, sulfur but alsophosphorous and calcium. In fact,calcium levels in seawater tripled during the early Cambrian.

“Calcium is what builds shells,” Harper explains. Over time, animals in the seasdeveloped thicker, more ornate shells. He says that’s one thing that “becomesvery, very obvious during the Cambrian Explosion.”

Such feedbacks would have exaggerated the pace of change. They also likelywould have led animal diversity to rapidly increase, Harper suspects.

The work by Harper and Smith shows us how closely Earth’s geology and biologyare linked. Tweak one, and you alter the other. In the strangeness of theCambrian, 540 million years ago, the results were as big as life itself.

Power Words

adaptation A process by which an organism or species becomes better suitedto its environment. When a community of organisms does this over time,scientists refer to the change as evolution.

Cambrian A period of Earth’s history that lasted from about 541 million to 510million years ago. It is one of the earliest periods in which fossils can be used todate rocks.


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diversity (in biology) A range of different life forms.

erosion The process that removes rock and soil from one spot on Earth’ssurface, depositing it elsewhere. Erosion can be exceptionally fast or exceedinglyslow. Causes of erosion include wind, water (including rainfall and floods), thescouring action of glaciers and the repeated cycles of freezing and thawing thatoften occur in some areas of the world.

feedback A processes or combination of processes that propel or exaggerate achange in some direction. For instance, as the cover of Arctic ice disappears withglobal warming, less of the sun’s warming energy will be reflected back intospace. This will serve to increase the rate of Earth’s warming. That warming mighttrigger some feedback (like sea-ice melting) that fosters further warming.

fossil Any preserved remains or traces of ancient life. There are many differenttypes of fossils: The bones and other body parts of dinosaurs are called “bodyfossils.” Things like footprints are called “trace fossils.” Even specimens ofdinosaur poop are fossils.

geochemistry A science that deals with the chemical composition of andchemical changes in the solid material of Earth or of another celestial body (suchas the moon or Mars).

habitable A place suitable for humans or other living things to comfortablydwell.

habitat The area or natural environment in which an animal or plant normallylives, such as a desert, coral reef or freshwater lake. A habitat can be home tothousands of different species.

halite A mineral formed from evaporation of seawater.

inclusion (in geology) Something trapped inside a mineral.

oxidation A process that involves one molecule’s theft of an electron fromanother. The victim of that reaction is said to have been “reduced.” It can makeitself whole again by robbing an electron from another molecule, triggeringanother case of oxidation. These chemical reactions are so violent, chemically,that they can easily kill cells. The oxidative reaction often involved oxygen atoms— but not always.

paleontologist A scientist who specializes in studying fossils, the remains ofancient organisms.

phosphate A chemical containing one atom of phosphorus and four atoms ofoxygen. It is a component of bones, hard white tooth enamel, and some minerals


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such as apatite.

phylum (plural: phyla) A group of living things. The modern animal kingdomincludes about 35 phyla.

Precambrian The period of geologic time that ran from around the time ofEarth’s formation, about 4.6 billion years ago, to 540 million years ago. During thisperiod, complex life — organisms containing many cells — emerged. The laterPrecambrian from about one billion to 540 million years ago, is also called theNeoproterozoic.

sulfate A family of chemical compounds that are related to sulfuric acid(H2SO4). Sulfates occur naturally in drinking water.

weathering The process of breaking down rocks and soil. Weathering can bechemical, such as by oxygenation (rust), or mechanical, such as by water, ice orwind.

Word Find (click here to enlarge for printing)


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S. Perkins. “Explainer: Understanding ice ages.” Science News for Students. Oct.17, 2014.

A. Yeager. “Lobsters’ ancient ‘cousin’ was gentle giant.” Science News forStudents. March 31, 2014.

K. Kowalski. “How Earth’s surface morphs.” Science News for Students. Aug. 7,2013.

D. Fox. “Watching our seas rise.” Science News for Students. Nov. 8, 2012.

S. Ornes. “One big animal family.” Science News for Students. Dec. 14, 2011.

J. Cutraro. “Salty, old and, perhaps, a sign of early life.” Science News forStudents. April 10, 2008.

K. McGowan. “How life made the leap from single cells to multicellular animals.”Quanta Magazine. August 1, 2014.

Original Journal Source: N. Planavsky et al. Low Mid-Proterozoic atmosphericoxygen levels and the delayed rise of animals. Science. October 31, 2014, p. 635.doi: 10.1126/science.1258410.

Original Journal Source: N. Planavsky et al. Evidence for oxygenicphotosynthesis half a billion years before the Great Oxidation Event. NatureGeoscience. March 23, 2014. doi:10.1038/ngeo2122.

Original Journal Source: T. Lenton et al. Co-evolution of eukaryotes and oceanoxygenation in the Neoproterozoic era. Nature Geoscience. March 9, 2014.doi:10.1038/ngeo2108.

Original Journal Source: Natalie Spear et al. Analyses of fluid inclusions inNeoproterozoic marine halite provide oldest measurement of seawater chemistry.Geology. January 6, 2014. doi: 10.1130/G34913.1.

Original Journal Source: M. Paul Smith and David A.T. Harper. Causes of theCambrian Explosion. Science. September 20, 2013. doi: 10.1126/science.1239450.

Original Journal Source: Don Canfield. Sulfate in the oceans. Proceedings of theNational Academy of Sciences. May 19, 2009. doi: 10.1073/pnas.0902037106.

Original Journal Source: Lee Kump. The Rise of Atmospheric Oxygen. Nature.January 16, 2008. doi: 10.1038/nature06587.

Further Reading


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