Life got bigger in two, million-fold leaps, scientists say

December 22, 2008,

( -- Extremes are exciting. Does anyone really think dinosaurs would capture our imagination the way they do if they hadn't been so huge? You don't see natural history museums vying for fossil skeletons of prehistoric rodents. It's the Tyrannosaurus rex fossils they salivate and squabble over. And would the Hollywood glitterati cart around those little teacup pups if they weren't so dang tiny and cute? Not likely.

Earth's creatures come in all sizes, yet they (and we) all sprang from the same single-celled organisms that first populated the planet. So how on Earth 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.

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

That might sound like a rather large undertaking, but, fortunately, the quest was made easier because even the professionals have a fascination with the size of the fossilized.

"The nice thing about maximum size is that people tend to remark on very large fossils, so they are much easier to track down in the geologic literature than anything else," Payne said.

In addition to quantifying the enormity of the two leaps in maximum size, the researchers also pinned down when those leaps took place. Both leaps coincided with periods when there was a major increase in the amount of oxygen in the atmosphere.

Payne said that many researchers already recognized, in a qualitative way, that the change in maximum size had occurred this way. "But our study really reflects the first time that anybody has tried to quantify exactly how stepwise it was and how big those size jumps were," he said.

A paper detailing the research by Payne and his colleagues is scheduled to be published in the Dec. 22, 2008, online early edition of the Proceedings of the National Academy of Sciences and is available online through EurekAlert.

The two other principal investigators of the research group, funded through the National Evolutionary Synthesis Center, are Michal Kowalewski of Virginia Tech and Jennifer Stempien of the University of Colorado-Boulder.

So how did it all happen? The first fossilized bacterial cells date to approximately 3.4 billion years ago, although life likely originated several hundred million years before. Between 2.7 and 2.4 billion years ago, cyanobacteria, formerly known as blue-green algae, originated and were of particular evolutionary and geological importance because they excrete oxygen as a waste product during photosynthesis. So far as science can tell, they were the first and only organisms to evolve oxygen-producing photosynthesis.

"All of the oxygen in the atmosphere ultimately exists because of the evolution of cyanobacteria," Payne said. "Plants that produce oxygen today during photosynthesis, their ability to do that is ultimately derived from cyanobacteria."

Single-celled bacteria remained the largest life form on Earth, cranking out the oxygen, until about 1.6 billion years ago. At that point, a new life form shows up in the fossil record.

"The first jump in maximum size happens when the first eukaryotic organisms show up as fossils," Payne said. "And those fossils are approximately a million times bigger than anything that had come before on Earth."

Although the first fossil eukaryotes were likely also single-celled organisms, the eukaryotes distinguish themselves by means of their internal structure and functioning. Instead of having the cellular processes of life take place by means of diffusion in the cell, eukaryotes have organized innards, with a nucleus and other cellular structures that are dedicated to specific functions in the respiratory process.

"The fossil record indicates pretty clearly that you need a eukaryotic cell to make that first size jump," Payne said. "It isn't just that the bacteria don't get there as fast, it is that bacteria still haven't gotten there 1.6 billion years later.

"Clearly, organismal organization matters," Payne said. "Not just at the time the size increase happens, but it continues to be a limitation on size.

For approximately the next billion years, life on Earth stayed about the same size, with only modest increases. Then about 600 million years ago, at the same time as another major boost in the amount of oxygen in the atmosphere, life leaped in size again.

This time, it was a million-fold size leap of multi-cellularity. Payne said there are clearly multi-cellular eukaryotes in the fossil record for several million years before this size leap, but the real explosion of size increase 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 first thing is that eukaryotic cells require oxygen for metabolism. So if they want to take organic matter and burn it up to have energy in their cell, they need oxygen. That sets the first and probably most important limitation."

Payne said this limitation also applies to multi-cellular eukaryotes, which likewise depend on extracting oxygen from the surrounding environment and using that in their cells to obtain energy. "There is also evidence that oxygen may mediate some other biochemical processes," he said.

As for just what triggered both the boosts in atmospheric oxygen, Payne said that isn't quite as clear. It may be that the first jump in oxygen came because cyanobacteria simply proliferated to the point that they were cranking out more oxygen than could be consumed through chemical reactions with material at Earth's surface, the only way that oxygen wouldn't have been released back into the atmosphere in the era before oxygen breathing creatures existed.

The possible causes of the second jump in oxygen are less clear, Payne said, but regardless of the puzzles that remain to be sorted out, the timing and magnitude of the jumps up in maximum size are clear. And Payne said the size jumps applied to a vast number of species.

"Whatever is controlling this second size increase appears to operate across many different groups. It is not something limiting one group alone," 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."

One other question remains to be answered: Can we look forward to another great leap in size? Will we see housecats larger than our houses?

"We've speculated on that a little bit, just sort of thinking about what if you went up another step," Payne said.

"The next level of organization, going along this kind of theme, presumably would be something like insect societies, where you have individual multicellular eukaryotes that specialize in terms of what kind of function they carry out in a larger organization of these individuals. Something like an ant colony or a human society would be in some ways the next organizational level.

"But, if you look at human society as an example, we use so much of the gross primary productivity on Earth, it doesn't appear there would be room for a lot of species at that next level of organization and maximum size. At that point you're actually getting towards the physical size limits just imposed by the size of our planet."

Provided by Stanford University

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4.4 / 5 (5) Dec 22, 2008
"physical size limits just imposed by the size of our planet."

If only we were surround by like, vast, empty space as far as the eye can see.
2.1 / 5 (7) Dec 23, 2008
Carbon in the Atmosphere

This has been a subject which has interested me for at least 40 years and prompted me to study for my first degree. I was originally looking at evolution of size as a component of partial pressure of oxygen. That thought lasted about five minutes when it became obvious that there is a more important limiting factor. If the partial pressure of CO2 is above a certain level, that amount of CO2 at least will remain in animal tissue, thus preventing growth and metabolic development, no matter how high the partial pressure of O2. My focus then change to atmospheric carbon and how that affected size and metabolic rate in animal life.

However, my interest is in the longitudinal development of the production and then the removal of carbon into and from the atmosphere. Its production would mainly be through volcanic activity releasing CO2 into the atmosphere and its removal would mainly be via photosynthesis. I developed a theory that looks at how the longitudinal development and removal of atmospheric carbon has taken place following a well known pathway in chemistry. Looking at the atmosphere as a result of a continuous chemical reaction, which looks "stable" at any point in time, I felt that over time it must follow a growth curve. To prove this I used CO2 and O2 as the two primary indicators in the %u201CEarth%u201D chemical reaction. In addition, the atmosphere being the main reservoir of those two active gasses is also where their respective partial pressures, in my theory, are the limiter and promoter respectively of metabolic rate. I used certain events in the geological fossil record as markers to indicate the points of change along the developmental curve. The relationship between the two gasses, and hence the metabolic rate supportable, is expressed by their relative partial pressures at any particular point in time.

To make this work I researched the origin in the animal fossil record of where qualitatively different metabolic rates occurred. My assumption here was that animal life would evolve into new areas of opportunity sooner rather than later by taking advantage of the ability to move faster, be stronger and utilize new environments previously inaccessible, in a purely opportunistic manner. This would happen only when a constraining factors on the metabolic rate were relaxed.

The minimum necessary and sufficient conditions to allow and maintain a higher metabolic rate would be a low enough CO2 partial pressure AND a high enough O2 partial pressure. This is the reason why the growth curve is an appropriate tool to describe the development of animal life - it describes the results of the %u201CEarth%u201D chemical reaction, it also happens, coincidentally, to chart, at any given point in geological time, where the leading edge of the metabolic rate for animal life on the planet would be.

It is now possible to specify the parameters of the growth curve. The curve must extend the whole length of Earth's history. It must show a lag phase where the ratio between the two gasses would show little or no change. Here the creation of O2 would be more than compensated for by the production of CO2 due to massive volcanic activity. A log phase where there was a rapid change in the two gasses, and if it developed that far, a stationary phase then a decline or death phase. The stationary phase would return to a situation where there would be little change in the relationship between the two gasses, but at a different level to that in the lag phase. The decline or death phase would reflect a reversal in the relationship between the partial pressure of the two gasses to a dangerous level, where the metabolic gains made during the log phase would start to unravel.

The lag phase is the Pre-Cambrian, extending way back to 4 billion years ago, with plant life coming onto the scene about 2 billion years ago. Animal life existed during some of this time but it is difficult to say when it started. All the various plant forms utilised the CO2 producing, among other things, O2. Animal life used O2 to make its metabolism of food work. That metabolism creates CO2 and in some animals, to rid themselves of the gas, which otherwise would build up in their tissue, they use Ca cations plus another oxygen atom, along with the CO2 all combining to produce a particle of calcium carbonate. Sufficient particles would provide an irritation within the animal's tissue and be pushed to the margins. The particles fall to the sea floor and form limestone, or, in deeper water disassociate into its component parts.

As the level of atmospheric CO2 falls and O2 increases the amount of CO2 that can be liberated by the animals' tissue increases, hence, causing more calcium carbonate to be produced. However, the animals' ability to remove calcium remains the same so eventually a build up of calcium in and around the animal is inevitable. These build ups reflected the shape of the animal and became the first calcium based fossils, and the start of the Cambrian. They also mark the end of the lag phase of my growth curve.

At the beginning of my log phase of the development curve, animal metabolism sees advances with the appearance of fish type animals, and at its end, the appearance of birds. The whole of the log phase of the development of metabolic rates happens over a 250 to 300 million year stretch. What supporting evidence is there in the geological record for such a change?

The evidence is in the amount of carbon taken out of the atmosphere over that 250 million year. Over this period there is massive deposition of CO2 in many forms. Its cumulative reduction and the cumulative increase of O2 via photosynthesis, drives the rapid change in metabolism over this period by reducing the partial pressure of CO2 and increasing the partial pressure of O2. Thus, a) allowing the release of greater amounts of CO2 from animal tissue, and, b) providing the O2 necessary to support the faster rates of metabolism. These cumulative changes in the gasses are reflected in the line of the growth curve during the log phase. The log phase line does not represent a smooth progression. In my opinion, reversals in the general trend happen as a result of increased volcanic activity and/or the loss of plant life. Reversals result in the partial pressures, particularly of CO2, changing to the detriment of removal of the gas from animal tissue. In these periods of reversals, those animals that have evolved to be the most advanced in terms of their metabolic rate OR bulk are the most at risk. They can no longer eliminate CO2 from their tissue at the rate demanded by their metabolic rate.

The result is an increase in acidity due to the trapped tissue CO2, possibly of pharmacological proportions. This lowering of the pH causes the genetic coding found in the cells in this particular tissue to become more unstable than the bodies waste control systems can cope with. The end comes from genetic disorders. This theory suggests that mass extinctions happen in the risk groups described, by the method described. Extinction events mark reversals on the growth curve, but progress later recovers with changes in the partial pressures of the two gasses getting back on course, leading to environments where even higher metabolic rates are possible and further qualitative changes to animal life happen.

The log phase ends when changes in the partial pressures of the two gasses slow and there no further qualitative changes to animal life are recorded. The stationary phase has begun.

In my terms the stationary phase started about 100 million years ago. This phase is marked purely by progress within animal life forms currently existing rather than any qualitatively new animal life forms, in terms of higher metabolic rates, evolving.

Recent human activity has forced an end to the stationary phase and the decline or death phase has begun. While the CO2 levels were decreasing, all previous life forms were able to enjoy an atmosphere which permitted the necessary removal of the waste product from their tissue. However, in the last 500,000 years, humans have had a growing effect on the %u201CEarth%u201D reaction by producing more and more CO2 at the expense of the mechanisms which balance that reaction.

By increasing the partial pressure of CO2 over this period, Man has brought %u201CEarth%u201D to a point where it can no longer continue to support the highest metabolic rates in animal life, or lower metabolic rates where the mass of tissue has evolved without the mechanism to remove CO2 as efficiently as required in the changed circumstances. The result is the lowering of tissue pH. This higher acidic environment allows a higher error rate during DNA replication. The body%u2019s mechanisms for clearing damaged protein have not evolved quickly enough to eliminate this damage. Genetic flaws abound in tissue and will kill more and more, possibly to the extinction of whole groups of animals, leaving only those with very low metabolic rates untouched.

This theory is testable. It can be considered from either the geological or biological bases of its evidence. I am asking that it is tested to destruction, and quickly because of its inescapable conclusion that life as we know it on this planet will change faster than we believe possible due to Global Warming, which is bad enough, but if increased partial pressure of CO2 is endangering higher metabolic rates, then we might already be too late to save ourselves.


1 / 5 (3) Dec 23, 2008
yeah, so maybe people will finly stop saying that evolution is gradual. Geeze we are a little bit past darwin here, people. It was a good first step, but stop quoting him and pretending you know jack about evolution.
not rated yet Dec 23, 2008
I would say evolution taking place over 1,000 years is definitely gradual. Let alone taking place over hundreds of millions of years...
2.3 / 5 (3) Dec 24, 2008

Thank you for your comments

Have there been any studies into the relationship between CO2 pp and metabolic rate? If so then they could knock this theory on the head and I could get on with the rest of my life! If not, it maybe an interesting place to start.

The errors in DNA replication are happening in the pheonotype, killing the individual before it is able to reproduce.
not rated yet Jan 09, 2009

Thank you for your positive comments.

You are right, more work needs to be done on this. However, I am not in a position to move it forward having retired. Also I know of no one likely to test out the historical accuracy of my ideas beyond the work I have done so far. The ideas on the relative effects of the two gasses came about after my studies into the geological timeline and fossil evidence.

The idea that CO2 was important came to me when I realised that the main change in the evolution of animal life was metabolism. I asked myself what would be the drivers, came up with CO2 pp reduction and O2 pp increase and looked back into the geological record for supporting evidence, such as the deposition of carbon in quantities sufficient to lower CO2 pp thus permit higher metabolic rates. The evidence is there in the geological record, massive depositions of carbon and changes in metabolism are found one following on from the other.

Have you any ideas on who could take his forward?

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