27 April 2012
by Anthony King
Evolutionary biologists have often pondered why life suddenly exploded into different forms 543 million years ago. Now, it seems seawater held the missing ingredient.
WHILE RIDING OVER THE Canadian Rockies in 1909, veteran geologist Charles Walcott discovered fossil gold among half-a-billion-year-old shale. These rocks contained evidence for all sorts of remarkable animals, and charted in great detail one of the enduring mysteries of evolution – the Cambrian explosion.
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| Trilobites were successful creatures during the Cambrian Explosion. |
The Cambrian period began around 543 million years ago. Rocks from the time before that offer poor territory for any fossil hunter. There are few fossils and what is there is mostly microscopic or can be placed in the “could be a fossil” drawer.
The Burgess Shale – around
505 million years old – belongs to a different era.
Once you enter the Cambrian, you witness a sudden explosion of animals in the rocks. All sorts of exotic marine creatures appear, but also recognisable relatives of
worms, arthropods and even
vertebrates.
This sudden appearance of allied species troubled Charles Darwin, who could give no satisfactory answer to this “grave difficulty,” as he described it, in his book On the Origin of Species. Why would life suddenly proliferate? Where were their ancestors?
SCIENTISTS HAVE SINCE IDENTIFIED good candidates of early animal life before the Cambrian – from the Ediacaran Period. It gets its name for the Ediacara Hills in the Flinders Range of South Australia, where famous fossils from this time where found.
From that time, what we find consists mainly of evidence for soft-bodied organisms lying peacefully on algal mats; some it seems were grazers, while others were filter feeders, but many possibly just lived in symbiotic partnerships with the mats they were sitting on. Generally everything was quiet and peaceful, says palaeobiologist Peter van Roy of Ghent University in Belgium. So what happened to this tranquil, if dull world? What set off a period of innovation in shells, plates, spines and other skeletal elements?
Van Roy believes the entrance of a new profession into the food chains of these tranquil oceans – predation – had major repercussions. “If you introduce predators in such a system, these soft, docile creatures of course make an easy lunch. So animals need to protect themselves from predation.” Creatures are pushed into developing defensive behaviors and other ways to protect themselves – this is where hard mineralised shells, exoskeletons and armour come into play.
The predators can counter this move by developing sturdy, possibly mineralised implements. “Prey, in turn, has to respond to these new challenges, so you end up with a classical arms race between prey and predators,” says Van Roy.
It had been hotly contested whether a slow build of genetic traits set the ball rolling or whether a trigger from outside lit the fuse for the explosion of diversity. The idea that factors within ecosystems were responsible gained the upper hand over the last decade or so. Now, though, US scientists have come along and upended the apple cart.
US SCIENTISTS REPORT evidence of what ignited the Cambrian explosion of life. The
chemistry of the seawater changed dramatically, they say, supplying a glut of raw material for shells, armour, exoskeletons, levers and mineralised parts. In support of their hypothesis, geologists
Shanan Peters of the University of Wisconsin and
Robert Gaines of
Pomona College, California, reported in the journal
Nature that what happened is evident in the “
Great Unconformity,” a puzzling and very substantial gap in the sedimentary – and hence fossil – record in many locations. In effect, the approximately
525 million year old Cambrian rocks rest on much older rocks.
“It is better called the great non-conformity,” says Peters, “because it juxtaposes two different types of rocks.” The sequence was named by an explorer navigating the Grand Canyon for the first time. Here, near the base of the canyon, he noted a dramatic switch in rock type. Rocks below a line are crystalline and hard, formed within the Earth – igneous and metamorphic – while almost a mile of rocks above the line are layered sedimentary rocks deposited from the Cambrian on.
THE GREAT UNCONFORMITY records a transition from a world where the continental surfaces were being eroded and weathered over vast areas of the planet (below the line), to a world where the seas flooded back onto the continents and marine sediment once again began to accumulate. The pre-existing rocks broke down at the exposed surface over millennia, explains geologist Patrick Orr of University College Dublin, Ireland. It was primarily the residue that this produced that was flushed into the oceans as sea levels rose and flooded back onto the continents, he explains.
Peters and Gaines argue that the exposure and chemical weathering of these rocks released the materials which later fuelled the evolution of hard parts such as shells and armour.
Freshly exposed rock weathers chemically at rates more than three times faster than undisturbed soils, freeing up chloride, magnesium, iron, potassium, sodium, carbonate and calcium ions. Peters, and other geochemists, believe calcium levels may have been so high in the ocean that it posed a challenge for animal life, interfering with cellular functions. Converting ions of calcium into a mineral such as calcium carbonate, however, would put them out of harm’s way. Once you start precipitating a hard mineral in this scenario, evolution has something to work with.
“We argue that biomineralisation didn’t evolve for claws and things like that; it evolved in response to a change in ocean chemistry,” says Peters.
“Once there was an initial metabolic reason to make a biomineral, of course natural selection then could use it as a tool and that gets pushed rapidly by ecology. The functional capability afforded by biomineralisation gets picked up very quickly by natural selection as an advantage and that gives us the Cambrian explosion and the diversity and morphology that we see.”
It is possible, argues Peters, that before the weathering of the continents the supply of chemicals was too low and so it was expensive to make biominerals. Once you have biominerals, it becomes possible and advantageous to evolve shells, eyes and other parts made of calcite and ultimately bones.
TRILOBITES FOR INSTANCE were among the most successful of early animals. These arthropods – in the same group as insects and crustaceans – had hard exoskeletons made of calcite (calcium carbonate) minerals along with the protein chitin. An array of trilobites roamed the seas and crawled along the seafloor, watching their Cambrian world through remarkably capable calcite eyes.
What they were looking out for were perhaps hunters such as Anomalocaris, the bizarre apex predator of the Cambrian seas. This is believed to be the proud owner of the amazingly complex 515 million year old eyes reported last year from the Emu Bay Shale of South Australia. Each eye consisted of at least 16,000 individual lenses, rivalling the best eyes in modern arthropods in terms of sight. Improving eyes among prey and predator is a sign of one-upmanship in an evolutionary arms race.
Van Roy agrees that biomineralisation could have been one of the changes that set off the Cambrian explosion, arming predators and prey. However, Nick Butterflied of the University of Cambrige doubts that changes in ocean alkalinity could have acted as a trigger for the Cambrian explosion.
BIOMINERALISING ORGANISMS, HE ARGUES, represent a trivial percentage of marine diversity and especially in the Cambrian. “The Cambrian explosion would have happened even in the absence of biomineralisation” he believes. His views offer a foretaste of the battle to come.
“The explosive radiation of biomineralization organisms in the early Cambrian wasn’t just about calcification,” he says. “Skeletalisation had very little to do with ocean alkalinity and a lot to do with ecology.” Butterfield notes that carbonate biomineralisation was an innovation in the Ediacaran, well before the “great unconformity.” The trigger he believes is the evolutionary appearance of animals, which sets of a cascade of unprecedented shifts in ecosystem function and expression – including biomineralisation.
The pendulum had swung toward intrinsic ecological factors as driving Cambrian evolution, such as predator-prey relationships, but this paper marks a clear wakeup call that environmental factors must be considered too as drivers of evolution, says Orr. Moreover, the time before the Cambrian teemed with creatures too, but without durable minerals these animals were ill-quipped to survive in forms that could be collected by palaeontologists, he explains.
To muddy the waters further,
many experts doubt that a single event caused the Cambrian explosion. Suggestions as to what caused or at least contributed to the explosion include
- an increase in the oxygen level in the atmosphere to a level that would sustain large and complex organisms,
- changes in oceanic microplankton,
- the development of visual organisms and
- the rapid continental movements leading to large methane releases.
In their recent Nature paper, the US researchers point out that an expansion of shallow sea areas coincided with the Cambrian explosion, offering bountiful conditions for life to thrive.
“It is probably a combination of some of the various suggested mechanisms,” says Jim Jago, Cambrian expert at the University of South Australia. “It should also be remembered that the Cambrian ‘explosion’ took place over a period of at least 30 million years in the bottom part of the Cambrian.”
It is likely that various biological and non-biological factors influenced each other, some possibly triggering others in some cascade, and possibly reinforcing each other, agrees Van Roy. “The Cambrian explosion is a complex event that cannot be explained by a single trigger. You have several factors – biotic and abiotic – influencing and potentially mutually reinforcing each other.” Such a complex set of pieces to an evolutionary puzzle, judged from a distance of 500 plus million years, offers fertile ground for lively debate for years to come.
