Organismal size over evolutionary time is a constrained stochastic property

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The intelligent design creationists are jubilant — a paper has been published that shows that organisms were front-loaded with genes for future function! It describes "'latent' or 'preexistent' evolutionary potential" in our history, they say.

One small problem. The paper says nothing of the kind. It does mention latent potential, but it means something entirely different from something that is 'front-loaded', which is a sneaky little elision on the part of the creationists. There isn't even the faintest whiff of a teleological proposal in the paper at all, which makes me wonder if they even read it, or if, as seems more likely, they're simply incapable of comprehending the scientific literature.

So let's take a look at what the paper is actually about, and you'll see that it in no way supports the self-serving cheering of the creationists.

First, though, a little background that will be familiar to many of you, especially if you've read Gould's Full House: The Spread of Excellence from Plato to Darwin(amzn/b&n/abe/pwll). The process we're looking at is the increase in size and complexity of life over time, a subject near and dear to the heart of creationists who see it as a predestined pattern driven by intent, but also of great interest to legitimate biologists, who have found that no, it is not purposeful at all, but a natural consequence of chance variation.

Here's an analogy to get the right model into your head. Imagine a busy bar that closes at 2am, and sends all the drunks out the door to walk home. Since scienceblogs was so unfair to our Australian readership last night, let's imagine it is an Australian bar, and a million brain-blitzed Australian drunks spill out the door and start walking determinedly down the street. There are a few properties at play here. One is that this street happens to be paralleled on the right by a wall, so the drunks can't stagger too far in that direction. The other is that on the left is a wide-open sheep pasture which provides no obstacle to their progress that way. Another is that they are all initially aimed straight down the street, but because they are drunk, they stagger every once in a while and veer off a few degrees to the left or the right, entirely by chance.

You're hovering overhead in a helicopter. What do you think you will see?

The mob will proceed down the street, but as it goes, it will spread out gradually to the left. The majority will stagger right and left with equal frequency, and wobble roughly down the street. There will be a subset that will, by chance, stagger left a little more than to the right, and they'll drift off into the sheep pasture. Some may veer more to the right than the left, but they'll just bounce into the wall and get straightened out that way.

No drunk Australian has a preference to stroll into the sheep pasture. There is no intent to end up there. But some do, just by the odds. You, in your helicopter, can even look at the shape of the sprawling mob and make useful calculations about drunk Australian kinetics and make predictions about the aggregate trajectories of strolling drunkards, although you wouldn't be able to predict the pattern of an individual drunk.

This is the general model for how size and complexity vary over time. The direction of the street is time, and wandering out farther and farther into the sheep pasture is like getting larger or more complex. There are other details that the analogy does not cover, however. There may be advantages to wandering to the left — it gets you out of the crowded mob of stumbling drunks. There may be intrinsic factors that limit how far a lineage can drift to the left, as well. Insect respiratory systems, for instance, create a kind of internal wall that limits how large they can get. There may also be external barriers that can be discerned by looking at the shape of the expanding mob.

Hop back into the helicopter and look down. What if the left edge of the mob doesn't expand exactly as you mathematically expected? What if it stops at some other barrier not visible from your vantage point? Perhaps there is a population of venomous Australian crocoducks lurking out there, or roving herds of carnivorous wallabies, and although we can't see them from up here, we do see that the drunks don't expand beyond a certain point.

Now that is the point of this new paper by Payne and others. They have taken a big picture examination of the distribution of fossil sizes over Earth's history, and asked whether the range has varied smoothly over time, as you'd expect if the outer bound were simply diffusing to higher levels. And the answer is no, it is not, there are a couple of discrete jumps in the maximum size that imply limits to earlier expansion that were overcome at specific periods in history. There was a wall of some sort to the left of the staggering mass of life, and they speculate a bit about what it might have been.

Here's the summary diagram of the results. The log of the volume of the largest fossils identified for a period are plotted on the Y axis, against time on the X axis. It is not a smooth curve, obviously — there are two large upward lurches, one about 2 billion years ago and another over half a billion years ago.

trend.jpeg
(Click for larger image)

Sizes of the largest fossils through Earth history. Size maxima are illustrated separately for single-celled eukaryotes, animals, and vascular plants for the Ediacaran and Phanerozoic. The solid line denotes the trend in the overall maximum for all of life. Increases in the overall maximum occurred in discrete steps approximately corresponding to increases in atmospheric oxygen levels in the mid-Paleoproterozoic and Ediacaran-Cambrian- early Ordovician. Sizes of the largest fossil prokaryotes were not compiled past 1.9 Gya. Estimates of oxygen levels from Canfield and Holland are expressed in percentage of PAL. Phan., Phanerozoic; Pz., Paleozoic; Mz., Mesozoic; C, Cenozoic. Red triangles, prokaryotes; yellow circles, protists; blue squares, animals; green diamonds, vascular plants; gray square, Vendobiont (probable multicellular eukaryote).

How do we explain these sudden upward surges in maximum size? Unsurprisingly, it isn't by postulating a being of unimaginable magical or technological power who visits Earth at that time and inoculates his chosen species with size boosting genes. There is absolutely no evidence for that, and no need to invent such a silly hypothesis. Instead, there are two very good explanations that are actually supported by measurements and observations (and strangely, scientists prefer those kinds of explanations). One is a change in the environment, and the other is an intrinsic change in a subset of life.

The environmental change is illustrated in the chart. The periods when the maximum organism size increased are correlated with periods when free oxygen levels in the atmosphere increased. Basically, the atmosphere was modified by the byproducts of organic metabolism in a way that allowed aerobic organisms to grow to a larger size — Earth accumulated enough rocket fuel in its atmosphere that some organisms could use to burn and grow. This is actually a fairly old story; we've been teaching about the oxygen increase in introductory biology for at least the last decade that I've been doing it.

The other part of the explanation is the one that has made our poor confused creationists so giddy, I'm afraid. The authors say,

These size steps coincide with, or slightly postdate, increases in the concentration of atmospheric oxygen, suggesting latent evolutionary potential was realized soon after environmental limitations were removed.

Ah, the realization of latent evolutionary potential. Did you know that you have latent evolutionary potential? Sure. If we put you and your family and friends in a novel environment, and let the generations tick by, we'll discover that certain sets of traits will become more prominent as selection and drift take their toll. The phrase does not imply that there is a purposeful arrangement of genes in your body that are there with a preexisting intent to allow you to thrive in a particular situation. You are complex, you have many properties that may in your current situation be superfluous or useless, but could be utilized in different situations.

You see, we have a good idea of exactly what intrinsic capabilities contributed to the 'latent potential' that led to certain lineages growing larger at those two transitions, and they both have natural precursors. We don't need a designer to explain the shifts, because the changes are expressions of known properties!

The abrupt increase in the Paleopterozoic, for instance, is the product of growth in size of the relatively recently evolved eukaryotes. You've probably heard of the endosymbiote hypothesis: eukaryotes are the product of a merger of multiple prokaryotic organisms into a single whole. Single celled organisms combined, with different specializations — organelles in our cells called mitochondria, for instance, are thought to be descendants of an incorporated prokaryote. Our mitochondria have the primary function of burning carbon and oxygen to produce energy. This was "latent evolutionary potential" that could be exploited by eukaryotes as oxygen availability rose. And so, following the rise of increasing oxygen concentrations, the size of some eukaryotes staggered upwards to a new maximum.

The second surge in the beginning of the Phanerozoic was also a consequence of a property with precursors in the existing single-celled populations: multicellularity. This is another biological property rich in "latent evolutionary potential". Again and unsurprisingly, simply combining multiple cells into one organism is a fast-track to larger organismal size, and we see multiple lineages exploring this capability, many of which failed and died out, such as the Ediacaran fauna, and others in the Cambrian that expanded rapidly. Multicellularity itself is not an abrupt, binary choice. We have precursors: modern choanoflagellates show that protists can find selective advantage in transient assemblies, colonial organisms show the virtues of more permanent arrangements, and creatures like sponges exhibit cooperativity and specialization in internal function. Chance creates the potential, and selection can drive an agency-free promotion of greater expression of that potential.

I must emphasize that this is not a paper endorsing any form of intelligent design creationism, and the creationists' appropriation of its conclusion depends entirely on their distortion of its contents. Here is the authors' full conclusion.

Although increase in maximum size over time can often be accounted for by simple diffusive models, a single diffusive model does not appear capable of explaining the evolution of life's overall maximum size. Approximately 3/4 of the 16-orders-of-magnitude increase in maximum size occurred in 2 discrete episodes. The first size jump required the evolution of the eukaryotic cell, and the second required eukaryotic multicellularity. The size increases appear to have occurred when ambient oxygen concentrations reached sufficient concentrations for clades to realize preexisting evolutionary potential, highlighting the long-term dependence of macroevolutionary pattern on both biological potential and environmental opportunity.

The way the creationists have abused that is by pretending that this implies that the evolution of the eukaryotes and of multicellularity had to have been purposeful events. This is simply not true. All this is saying is that the limits of growth are properties of both organismal and environmental constraints, and that we can map those out by looking at the fossil record.


Payne JL, Boyer AG, Brown JH, Finnegan S, Kowalewski M, Krause RA Jr, Lyons SK, McClain CR, McShea DW, Novack-Gottshall PM, Smith FA, Stempien JA, Wang SC (2009) Two-phase increase in the maximum size of life over 3.5 billion years reflects biological innovation and environmental opportunity. Proc Natl Acad Sci U S A. 106(1):24-7.

17 Comments

This is really cool. I love this sort of analysis demonstrating large-scale systematic relationships between environmental change and evolution. Thanks for posting it.

Sigh.

If there is any possible way to misinterpret something no matter how small, ID/Creationists will find it.

If there is no possible way to misinterpret something, ID/Creationists will invent it.

Now that is what front-loading is all about.

Leave it up to ID proponents to not understand the science, which is probably why the scientific impact of ID is exactly _zero_

Sigh, the teleological argument is the closest thing in the world to perpetual motion:

http://www.vectorsite.net/tadarwin_20.html#m2

Cheers – MrG

You probably wanted to say:

The other is that on the left is a wide-open sheep pasture which provides no obstacle to their progress that way.

let’s imagine it is an Australian bar, and a million brain-blitzed Australian drunks spill out the door and start walking determinedly down the street.

I think I was at that bar the last time I was in Sydney, but I don’t remember much.

Over at the UD blog, a couple of people mentioned ID research to prove the front-loading. Strangely enough, its always someone else who will do the research.

let’s imagine it is an Australian bar,

Like the one in Crocodile Dundee?

Henry

Over at Chris Nedin’s blog, he predicted back in December that the creationists would quote-mine this paper!

Frank B Wrote:

Over at the UD blog, a couple of people mentioned ID research to prove the front-loading. Strangely enough, its always someone else who will do the research.

Michael Behe in fact proposed front-loading in “Darwin’s Black Box” way back in 1996, and to my knowledge has not taken the first step to test it.

I haven’t yet read how the DI spins it now, but Behe’s proposal was that the front-loading occurred billions of years ago in a cell that’s ancestral to all subsequent organisms. That of course would be no comfort to (common descent-denying) YECs and OECs even if front-loading had promise. The DI could conceivably challenge Behe’s version with a “many times in many ‘kinds’” alternative, possibly one that only occurred 1000s of years ago. IOW more OEC-friendly if not YEC-friendly. But I’ll bet that they did nothing of the sort, and rather resorted to “don’t ask, don’t tell when or where the loading occurred.”

One of the authors posted a reply on Pharyngula. For general interest I’ve copied his post (#137) below.

First I should probably introduce myself…I am one of the authors on the study (I also blog at Deep Sea New). I have to say I amazed, and never thought in my wildest dreams, that the paper would ever be used as evidence for ID. Let me say that it clearly DOES NOT do any such thing. PZ has done an excellent write up of what the paper actually shows and there is no need for me to reiterate any of it. Thanks PZ!

I will respond to a few of the comments

If it wasn’t for that stupid term “latent evolutionary potential”, this paper would not have received any attention at all. It is just a rehash of things we have known for years. and if it wasn’t for the authors using the Track I submission (submitting through a PNAS member) it never would have been published in PNAS. PNAS should eliminate that archaic Track I that lets boring stuff be published just bc the authors are buddies w a PNAS member. and I can think of no better way to begin that elimination than by commenting on a blog

Actually the paper’s acceptance and attention is because of its novelty. Indeed before this post I had not even remembered that we used the term latent evolutionary potential. What the paper does for the first time ever is quantitatively document how life on earth, through its entire history, achieved larger and larger sizes. It is not a rehash of things we already know. In actuality the paper provides both novel insights and changes the way we view body size evolution. For example, I believe that many biologists, including myself, would have predicted before the paper increases in size to be both more gradual and constant. The paper also show the importance of increases in organizational complexity and oxygen. Whereas the relationship of size and oxygen is both predicted and evidence exists for this in modern assemblages (including my own work), to my knowledge it has never been so intrinsically linked in the fossil record over such a broad scale. Moreover, as I recall none of us are ‘buddies’ with the editor and I will also add the paper was peer reviewed.

… Aren’t they forgetting that life also got smaller? I mean, humans aren’t exactly bigger than dinosaurs. Or did I miss something? (I’m also not a scientist and haven’t read the actual paper)

Indeed! As PZ mentions above under a diffusive, passive evolutionary model we expect the increases in maximum size, but we would expect decreases in minimum size. Over time the overall range in body size would increase, i.e. an expansion into the size evolutionary space. Here we only look at the largest species for each time interval. Given the vagaries of the fossil record, we are much more likely to accurately quantify and know the largest size. However, the smallest organism from each time interval is likely unknown or even unknowable.

Hi PZ,

Thanks very much for posting this. It is truly a most fascinating look as to how organismal body size responded to changes in the composition of Earth’s atmosphere during the first 4 billion years of our planet’s history. Am not surprised that organismal body size “exploded” after approximately one billion years ago, since this is approximately when oxygen levels in the atmosphere began to reach a level comparable to ours.

I am not surprised that Jim Brown of the University of New Mexico’s biology department participated in this study. He’s been interested in the evolution of body size in metazoans for decades, and has thought quite clearly about this (On a personal note, I remember him as a kind and quite inspirational graduate school professor at the University of Arizona.).

Appreciatively yours,

John

Its amazing that creationists cant understand unintended phenomena, chance and natural selection. Thats not their fault, Its their brain which lacks well functioning! And isnt it an evolutionary phenomena?!!

PvM Wrote:

Leave it up to ID proponents to not understand the science, which is probably why the scientific impact of ID is exactly _zero_

Given their prior commitment to misrepresenting evolution, and doing whatever is necessary to keep all “kinds” of evolution-deniers under the big tent, if they did understand the science, wouldn’t their impact still be zero?

Hmm. That the state of the process of evolution may visibly move when constraints shift isn’t IMHO more of a showing of latent potential as is inherent in the dynamics of the process itself. So I concur with those who thinks it’s an unfortunate choice of description, but for other reasons.

Otherwise I’m pleased, since I recently learned of a bistability model for oxygen in atmospheric evolution which concur with these results. In essence, one can understand the first shift (both papers use Grypania is indicator) as occuring after the switch from very low oxygen levels in a methane/oxygen atmosphere to an atmosphere with a protective ozone layer.

Dunno about the later shift though. Is it indicative of the oxygen diffusion length in tissue, i.e. did the Ediacaran biota take advantage of a possibility to have multiple cell layers? If so, the first shift was due to an atmospheric constraint and the later a biological constraint.

a methane/oxygen atmosphere

D’oh! Containing methane and oxygen, that is.

Also, this reminded me of another pleasing correspondence between physical and biological constraints I recently noted, and a topic that may touch on the above.

Nature recently released an article on adaptations to high temperatures for early life, “Parallel adaptations to high temperatures in the Archaean eon” which ties in with the models of Archaea and Bacteria as reduced compared to an RNA LUCA, mainly due to thermoadaption. Both set of ‘thermometers’, (prokaryotic) proteome and rRNA, points to adaptation from a mesophilic LUCA to organisms capable of tolerating 70 - 80 Celsius.

And lo and behold, there are papers that get the Archaean oceanic temperature to 70 - 80 Celsius: “A palaeotemperature curve for the Precambrian oceans based on silicon isotopes in cherts”. (Again from two rather independent thermometers, btw.)

And now when I look at the later paper’s figure of oceanic temperature curves, I can’t but not notice that they too starts to drop at 2 Ga, moving down towards more modern temperatures.

Now, I assume both papers represents minority views in the two different scientific communities. But at least a layman can be fascinated by the possible correlations.

Torbjörn,

Can’t speak about the paper you’ve cited since it falls outside my interests and former area of expertise:

Torbjörn Larsson, OM said:

a methane/oxygen atmosphere

D’oh! Containing methane and oxygen, that is.

Also, this reminded me of another pleasing correspondence between physical and biological constraints I recently noted, and a topic that may touch on the above.

Nature recently released an article on adaptations to high temperatures for early life, “Parallel adaptations to high temperatures in the Archaean eon” which ties in with the models of Archaea and Bacteria as reduced compared to an RNA LUCA, mainly due to thermoadaption. Both set of ‘thermometers’, (prokaryotic) proteome and rRNA, points to adaptation from a mesophilic LUCA to organisms capable of tolerating 70 - 80 Celsius.

And lo and behold, there are papers that get the Archaean oceanic temperature to 70 - 80 Celsius: “A palaeotemperature curve for the Precambrian oceans based on silicon isotopes in cherts”. (Again from two rather independent thermometers, btw.)

And now when I look at the later paper’s figure of oceanic temperature curves, I can’t but not notice that they too starts to drop at 2 Ga, moving down towards more modern temperatures.

Now, I assume both papers represents minority views in the two different scientific communities. But at least a layman can be fascinated by the possible correlations.

But the PNAS paper - whose authors include ecologists and at least one paleobiologist - merely reaffirms what is known about how life responded to increasing oxygen levels, especially after one billion years ago.

Regards,

John

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This page contains a single entry by PZ Myers published on January 14, 2009 11:47 AM.

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