"Long before it's in the papers"
April 28, 2009

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Life got bigger in two, millionfold leaps, researchers say

Dec. 26, 2008
Courtesy Stanford University
and World Science staff

Earth’s crea­tures come in all sizes, yet sci­en­tists be­lieve they all de­scend from the same single-celled or­gan­isms that first pop­u­lated the plan­et. So how did life go from bac­te­ria to the blue whale?

“It hap­pened pri­marily in two great leaps, and each time, the max­i­mum size of life jumped up by a fac­tor of about a mil­lion,” said Jon­a­than Payne, a ge­o­log­ist and en­vi­ron­men­tal sci­ent­ist at Stan­ford Uni­ver­s­ity in Cal­i­for­nia.

Cyanobac­te­ria, the first oxy­gen-pro­du­c­ing or­gan­isms, may have cre­at­ed the en­vi­ron­ment nec­es­sary for size leaps by oth­er crea­tures, re­search­ers say. (Pho­to: J. Wa­ter­bury, Woods Hole/NA­SA As­tro­bi­ol­ogy Inst.)

Payne, with a doz­en pa­le­on­tol­ogists and ecol­o­gists at 10 re­search in­sti­tu­tions, pooled their databases, combed sci­en­tif­ic lit­er­a­ture and con­sulted with tax­o­nom­ic ex­perts in a quest to de­ter­mine the max­i­mum size of life over ge­o­log­i­cal time. 

Both leaps co­in­cid­ed with per­i­ods when there was a great in­crease in the amount of ox­y­gen in the at­mos­phere, Payne said. A pa­per de­tail­ing the re­search ap­peared in the Dec. 22 on­line early edi­tion of the re­search jour­nal Pro­ceed­ings of the Na­tional Acad­e­my of Sci­ences. 

The first fos­sil­ized bac­te­ria date to about 3.4 bil­lion years ago, al­though life likely arose sev­er­al hun­dred mil­lion years be­fore, Payne said. Be­tween 2.7 and 2.4 bil­lion years ago, cyanobac­te­ria, form­erly known as blue-green al­gae, orig­i­nat­ed and were of par­tic­u­lar im­por­tance be­cause they give off ox­y­gen in the pro­cess of draw­ing en­er­gy from light. Plants, which do the same thing, de­scend from cy­ano­bac­te­ria.

The single-celled bac­te­ria re­mained Earth’s larg­est life form, crank­ing out the ox­y­gen, un­til about 1.6 bil­lion years ago, Payne added. Then a new life form shows up in the fos­sil rec­ord whose max­i­mum size is about “a mil­lion times big­ger than an­y­thing that had come be­fore.” Those or­gan­isms are called eu­kary­otes.

Eu­kary­otes have cells of more com­plex struc­ture than bac­te­ria. Their cells con­tain a nu­cle­us and oth­er com­part­ments ded­i­cat­ed to spe­cif­ic func­tions. This shows “or­gan­iz­a­tion mat­ters” for size, Payne said. For bac­te­ria, the rel­a­tive lack of it “con­tin­ues to be a lim­ita­t­ion on size.”

About 600 mil­lion years ago, at the same time as anoth­er ma­jor boost in the amount of ox­y­gen in the at­mos­phere, life grew again, Payne went on. This time, it was a mil­lion-fold size leap of mul­ti­-cell­u­lar­ity. Payne said there are mul­ti­-cell­ular eu­kary­otes in the fos­sil rec­ord for sev­er­al mil­lion years be­fore this size leap, but the real ex­plo­sion did­n’t hap­pen un­til the ox­y­gen lev­el bumped up.

So why do the size leaps seem to hinge on the amount of ox­y­gen in the air? “There are a few things that could be go­ing on,” Payne said. The most im­por­tant is that eu­kary­otes need ox­y­gen for me­tab­o­lism. If they want to eat—in oth­er words, “take or­gan­ic mat­ter and burn it up to have en­er­gy... they need ox­y­gen.”

Payne said the first boost in at­mos­pher­ic ox­y­gen might have come be­cause of the pro­lifera­t­ion of ox­y­gen-generating cya­no­bac­te­ria. The causes of the sec­ond boost are less cer­tain, Payne added, but the tim­ing and mag­ni­tude of the jumps up in max­i­mum size are clear, and af­fected vast num­bers of spe­cies. 

“What­ever is con­trol­ling this sec­ond size in­crease ap­pears to op­er­ate across many dif­fer­ent groups,” he said. “There al­so ap­pears to be an in­crease even in the max­i­mum size of groups of or­gan­isms like mul­ti­-cellular al­gae, so the size in­crease does­n’t ap­pear to be lim­ited just to an­i­mals.” 

Can we look for­ward to anoth­er great leap in size? Probably not, Payne said, be­cause we hu­mans, for ex­am­ple, are al­ready strain­ing Earth’s re­sources at our cur­rent size. “Y­ou’re ac­tu­ally get­ting to­wards the phys­i­cal size lim­its just im­posed by the size of our plan­et.”

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Earth’s creatures come in all sizes, yet scientists believe they all descend from the same single-celled organisms that first populated the planet. So how did life go from bacteria to the blue whale? “It happened primarily in two great leaps, and each time, the maximum size of life jumped up by a factor of about a million,” said Jonathan Payne, assistant professor of geological and environmental science at Stanford University in California. Payne, with a dozen other paleontologists and ecologists at 10 different research institutions, pooled their databases, combed scientific literature and consulted with taxonomic experts in a quest to determine the maximum size of life over all of geological time. Both leaps coincided with periods when there was a great increase in the amount of oxygen in the atmosphere, Payne said. A paper detailing the research appeared in the Dec. 22 online early edition of the research journal Proceedings of the National Academy of Sciences. The first fossilized bacteria date to about 3.4 billion years ago, although life likely arose several hundred million years before, Payne said. Between 2.7 and 2.4 billion years ago, cyanobacteria, formerly known as blue-green algae, originated and were of particular importance because they give off oxygen in the process of drawing energy from light. Plants, which do the same thing, descend from cyanobacteria. The single-celled bacteria remained Earth’s largest life form, cranking out the oxygen, until about 1.6 billion years ago, Payne added. Then a new life form shows up in the fossil record whose maximum size is about “a million times bigger than anything that had come before.” Those organisms are called eukaryotes. Eukaryotes have cells of more complex structure than bacteria. Their cells contain a nucleus and other compartments dedicated to specific functions. This shows “organization matters” for size, Payne said. For bacteria, the relative lack of it “continues to be a limitation on size.” About 600 million years ago, at the same time as another major boost in the amount of oxygen in the atmosphere, life grew again, Payne went on. This time, it was a million-fold size leap of multi-cellularity. Payne said there are multi-cellular eukaryotes in the fossil record for several million years before this size leap, but the real explosion didn’t happen until the oxygen level bumped up. So why do the size leaps seem to hinge on the amount of oxygen in the air? “There are a few things that could be going on,” Payne said. The most important is that eukaryotes need oxygen for metabolism. If they want to eat—in other words, “take organic matter and burn it up to have energy in their cell—they need oxygen.” Payne said the first boost in atmospheric oxygen might have come because of the proliferation of oxygen-generating cyanobacteria. The causes of the second boost are less clear, Payne added, but the timing and magnitude of the jumps up in maximum size are clear, and applied to vast numbers of species. “Whatever is controlling this second size increase appears to operate across many different groups,” he said. “There also appears to be an increase even in the maximum size of groups of organisms like multi-cellular algae, so the size increase doesn’t appear to be limited just to animals.” Can we look forward to another great leap in size? Probably not, Payne said, because we humans, for example, are already straining the planet’s resources at our current size. “You’re actually getting towards the physical size limits just imposed by the size of our planet.”