The Science of Evolution

The Cambrian Explosion, Punctuated Equilibrium and the "God of the Gaps" Argument

by Ardea Skybreak

Revolutionary Worker #1217, October 26, 2003, posted at

What's wrong with the Creationists is not just that they make false statements about the way things are (such as the idea that all living species are completely unrelated and were all created separately and at the very same time just a few thousand years ago, or that humans lived at the same time as dinosaurs, and so on). These absurdities are bad enough, but what's even more wrong with the Creationists is their fundamentally incorrect approach and method for supposedly getting to the truth of things. This is important to understand because many Creationists have shown that they are perfectly willing to change their specific anti-evolution arguments, especially as scientists expose their lies and distortions and many in the general public become more wise to their smoke-and-mirrors charlatanism. Creationism actually seems to "evolve" (though obviously this is an example of cultural, not biological, evolution) as scientists and others pursue Creationists with the truth. The Creationists have a definite agenda which seeks to undermine secular science and replace it with practices and methods rooted in religious belief. Ironically, in order to accomplish this goal they know they need to try to blur the distinction between science and religion and get people to start thinking that religion is science. So these days Creationists such as the Intelligent Design types are sometimes willing to completely abandon the details of the traditional Biblical creation story in favor of a more scientific- sounding theory which claims to find evidence of divine design in, for instance, the ways molecules are assembled at the sub-cellular level. Many people who have no trouble understanding how absurd and ridiculous it is to think that all the species we see today could be the descendants of mated pairs Noah managed to shove onto his boat often don't know anything about biochemistry and molecular biology, so of course the Creationists are trying to take advantage of this ignorance.

Since the Creationists are willing to just keep changing their specific arguments whenever they find themselves a bit too much on the hot seat, you can't possibly expect to have ready answers to all their distortions. What you need to do is learn to identify their wrong METHOD because, unlike their specific arguments, their fundamental method (the way they approach trying to get to the truth of things) never really changes.

Consider, for instance, the question of gaps in human understanding. At any given time there are obviously going to be gaps in human knowledge--there are always going to be some things that we don't yet understand. The "God of the Gaps" argument refers to the typical (and very annoying!) Creationist method of trying to make their case for divine creation not by providing any "evidence" of divine creation (which of course they cannot do), but by pointing to various holes in current human knowledge and acting as if this in itself "invalidates" evolution and instead "proves" divine creation must have happened . They insistently demand that evolutionists immediately explain one or another aspect of the natural world that is in fact not yet fully understood (or that the Creationists are grossly distorting to make it look as if is not yet understood); and if someone can't do this, they dramatically throw up their hands and act as if there are so many unexplained puzzles that the theory of evolution cannot possibly be right, and that if it is wrong, the only possible alternative is that the story of divine creation must be right (among other things, there is an obvious flaw in their logic here: even if the theory of evolution were wrong--which it isn't--that wouldn't automatically mean the theory of divine creation is right!).

It is obviously important for people to try to educate themselves about what we now understand to be the proven facts of science. But it is also worth reflecting on the fact that many things which were, at one time, not yet understood or explainable, later on became completely understandable. Take the example of the so-called "mystery" of the Cambrian explosion:

The "Cambrian explosion" seen in the geological record is something Creation- ists often still like to bring up to argue that this is evidence of some kind of sudden and separate divine creation of complex life-forms. Their so-called "evidence" for this simply boils down to the fact that for a long time evolutionists didn't yet know how to explain it. So let's look at this a bit.

The so-called "Cambrian explosion" is certainly real. It refers to a time at the very beginning of the Cambrian geological era (some 540 million years ago) when, seemingly "out of nowhere and all of a sudden," the fossil record reveals that the seas became full of all sorts of complex multi-cellular animals which had never existed before, including corals, brachiopods, clam-like mollusks and the first of the arthropod trilobites.*

The fossil record clearly shows, for instance, that trilobites first appeared in this period (trilobites are creatures you could think of as somewhat resembling marine cockroaches) and then underwent a tremendous burst of species diversification. In fact, the many different species of trilobites ended up dominating the earth's seas for hundreds of millions of years (though all the trilobites are now extinct). Such bursts of extensive biological diversification are not seen in the fossil record prior to the Cambrian: if you look for fossils in layers of rock that are older than 540 million years you won't find any of these species; in fact you won't find many fossils of any species at all! Pretty much zip. But that doesn't mean that life on earth started only 540 million years ago. We know that life on earth started around 3.5 billion years ago because we can find traces in rocks of simple bacteria-like organisms going back at least that far. And fossil remains of the first more complex but still single-celled eukaryotes (cells with their DNA contained inside a nucleus instead of held just as loose strands in their bodies, as in the case of bacteria) have been found fossilized in rocks dating back as far as 2.5 billion years ago. But these are still one-celled organisms. Multi -cellular organisms, which are made of complex collections of eukaryotic cells which perform different functions, don't show up in the fossil record until the beginning of the Cambrian period. At that point, seemingly all of a sudden--"wow, there they are!"

So what's going on here? If evolution is real, these complex marine animals (like the trilobites) couldn't just have popped out of nothing. And they have too many components to have appeared as "instant" modifications of those previous one-celled organisms, because evolution just doesn't work that way. They had to have been derived from pre-existing ancestor species, but what were those ancestors and where are they? This "mystery" is one of the things Creationists for more than a century have been trying to use to say evolution must be wrong, showing once again that they offer no positive proofs of any of their arguments--their whole strategy is just to try to "poke holes" in the theory of evolution and hope it will collapse and divine creation will be the only theory still standing. The way the Creationists see things is that "if you can't explain something perfectly then god must have done it."

But today the so-called "mystery" of the Cambrian explosion (the seemingly "sudden" appearance of complex multicellular marine animals around 540 million years ago) is being solved on two fronts:

  1. scientists have been able to collect additional observational data , such as the discovery of the remains of a diverse collection of soft-bodied multicellular organisms in fossil layers older than 540 million years old, which likely include some of the ancestor species from which the Cambrian animal groups are descended.
  2. scientists have also made significant theoretical advances which are contributing to a better understanding of large-scale evolutionary processes spread out over the very long stretches of geological time (such as significant advances in the understanding of how major changes in climate or other environmental changes can "drive" bursts of speciation, or the 1972 proposal by Niles Eldredge and Stephen Jay Gould of the theory of "punctuated equilibrium," arguing that large- scale evolutionary change has happened at variable rates over the millennia and that the fossil record should reflect that unevenness). These kinds of theoretical contributions--and the rich atmosphere of fresh debate and energized research they have brought forth--have gone a long way towards resolving the supposed "mystery" of the Cambrian explosion.

As it turns out, it's not that multi-cellular organisms are completely absent from the fossil layers that are older than 540 million years old: it's more that they are rare , and so they are obviously hard (though not impossible) to find. Why are they hard to find in the fossil record? Well, for one thing, we now know something we didn't used to know: that the planet went through a whole series of profound and extensive glaciations (described as periods of "snowball earth") between about 600 and 800 million years ago, before the Cambrian "explosion." It doesn't seem these periods of such severe global glaciation would have been very favorable for sustained periods of evolutionary diversification (and fossilized preservation) of complex multi-cellular life forms. But it is also now known that at the end of the last of these "snowball-earth" glaciations (during which the entire planet had been covered with ice) multi-cellular organisms did manage to get an evolutionary toe-hold and from then on were able to spread and diversify quite rapidly.

We now know this because, in rock layers dated to between 540 and 600 million years old, remains of the so-called Ediacaran fauna have been found. These were (to our human eye) strange-looking and clearly multi-cellular animals. Their exact classification is still being debated, but many of them look like relatives of the later worms, corals, starfish and arthropods, and paleontologist Niles Eldredge for one is pretty sure they're going to turn out to be some of the direct ancestors of the Cambrian organisms which had previously seemed to "come out of nothing."

Keep in mind that one reason it's so difficult to find fossils of the oldest multicellular species is because the first multi-cellular creatures lived in water and were soft-bodied --without skeletons or other hard-parts--so they would often have decomposed without leaving any fossil traces at all. It has been suggested that, until about 540 million years ago, the ancient seas did not yet contain enough oxygen to allow the development of large multicellular organisms or even the production of materials like calcium carbonate that goes into making hard body parts. As often seems to be the case in the evolutionary history of the earth, major and large-scale changes in environmental conditions seem to have opened up new possibilities for life to evolve along dramatically new evolutionary pathways never before seen.

With increased understanding from a number of different directions we now realize why the fossil record prior to the Cambrian seems somewhat empty and barren, and we understand better why the so-called "explosion" onto the scene of a whole set of new multicellular hard-bodied creatures some 540 million years ago required no divine intervention and did not, in fact, "come out of nothing" but out of some of the pre-existing multicellular species which are only now being discovered.

Theoretical advances in evolutionary biology have also helped us to rethink how something like the Cambrian explosion occurs (because even though it didn't "come out of nowhere" it is still an "explosion" in the sense that it represents a tremendous, and geologically rather sudden, burst of new species, many of which would fill the seas for millions of years thereafter).

In fact, the Cambrian explosion is helping us to deepen our understanding of the variable rhythms and pacing of the kind of large-scale evolutionary changes that can take the branching bush of life into radically new directions. These processes can be relatively slow and plodding for tens or even hundreds of millions of years, and then really pick up in a concentrated "burst" of truly major evolutionary modifications of whole lines of plants or animals over maybe just a few hundreds of thousands or millions of years. The Cambrian explosion is an example of one such burst. While smaller- scale evolutionary modifications are always going on at or below the level of individual species (at the level where local reproductive populations of living species interact with their local environments), the truly large-scale and qualitative leaps in the history of life on this planet (such as the appearance and spread of the first insects or of the first mammals, for instance) are much more rare occurrences. It is being increasingly understood that macroevolutionary change is not stuck on a single speed like some antique bicycle: there have been times in the history of this planet when, in a sense, very little was happening in terms of the appearance of any radically new evolutionary directions, and such periods of "relative stasis" on the macroevolutionary level have been fairly typical (even while the existing species always continued to evolve) and have often lasted for tens of millions or even hundreds of millions of years. But the history of life on this planet has not been just one slow and gradual train: it's becoming increasingly clear that it has also been periodically "punctuated" by relatively short periods of time in which many really significant, large-scale evolutionary changes have taken place--such as the nearly simultaneous (on a geological time-scale) appearance and spread of many new and radically different families of plants and animals--out-of-the-ordinary and concentrated bursts of speciation and diversification which have repeatedly taken life on this planet into whole new directions.

Importantly, it is also becoming increasingly clear that those really major leaps and turning points in large-scale evolutionary history (the ones that gave rise to whole new branches and directions in the branching bush of life) have tended to correspond to periods of major change and dislocation of the existing physical environment (such as major changes in the physical landscapes and climate) and of the existing biotic environment (such as major changes in the collection of living--"biotic"--species of plants and animals that share the same environment and interact with each other in various ways). We know from direct experience and observation in both laboratories and in the wild that sustained environmental disruptions on even a more minor and "local" scale can drive living populations of plants and animals to begin to spin off distinct species even today (for instance, as was discussed earlier in this series, bacterial populations will often start to evolve new resistant strains when antibiotics are repeatedly introduced into their environments; or populations of one species of fish which once shared an environment can, over the generations, start to diverge into two different species if something like a sandbar or clogged waterway keeps two parts of the population separated and unable to reproduce and exchange genes with each other for a long enough period of time). The basic underlying evolutionary principles are much the same at both the "micro" and the "macro" evolutionary scales.

The early primitive single-celled bacteria seem to have done fine since the earliest days of life on this planet--in fact, it is often said that "bacteria still rule the earth" (and not just causing diseases; many bacteria species are also involved in producing oxygen in the seas, cycling nitrogen in the soil and breaking down dead wood and other organic matter--the rest of life as we know it would literally not be possible without them!). The first simple eukaryotic cells evolved about a billion years after those earliest bacteria, and they in turn became the building block ancestors out of which all later multi-cellular plants and animals were ultimately derived.

The evolution of multi-cellularity is a fascinating story in its own right. It turns out the very first multi-cellular organisms likely evolved as a result of some kind of aggregation or "fusing together" of different one-celled organisms which lived in the early seas. Today a complex body such as a human body contains something like 3 billion separate cells, many of which perform different specialized functions. But the marks of a simpler past in the history of life on earth are still visible today in every one of the cells of every single multi-cellular plant or animal, including people. This is because each individual cell contains little energy producing organelles, called chloroplasts (in the case of plants) or mitochondria (in the case of animals, including humans). These critical little energy producing factories found inside of cells allow them to engage in much more complex and diverse functions than would be possible in one-cell bacteria, for instance. But it turns out (as Lynn Margulis first insightfully proposed in 1967) that those chloroplasts and mitochondria actually used to be separate free-living creatures themselves ! At one time, some of them apparently fused with other early cells in some kind of symbiotic or parasitic relationship...and the rest is history!**

Talk about a major change in a biotic environment (the external environment which includes all the other living species in a locality) leading to a whole new spin-off branch in the evolution of life! But one could point to many other significant examples: for instance, the accumulation of enough oxygen in the seas which would have allowed the production of the calcium carbonate that some organisms then used to make shells and other hard parts seems to have spurred yet another major evolutionary "burst" of diversification; a major asteroid impact 65 million years ago seems to have significantly contributed to the extinction of the dinosaurs and the rapid expansion and diversification of mammals; and, in our own history, major environmental changes in Africa (in particular a series of serious and long-lasting droughts which would have significantly altered the hominid environments and food sources) seem to have given impetus to a number of significant developments in the evolution of the various human species which were the ancestors of our current single human species. As Niles Eldredge put it:

"Almost without fail, whether we are considering ancient bacteria 3.8 billion years old or the evolution of the human lineage in the last 4 million years, we find that significant events in life's history are correlated with significant events in the history of the Earth's atmosphere, oceans, and lands."

The Cambrian explosion, our own human evolution, or any other major junctures in the evolutionary history of life, can be explained by entirely natural processes and require no super-natural explanations. But will our ability to increasingly understand life without god leave us feeling in some way adrift and empty? Why should it? We still can, and should, be duly amazed, sustained, and even humbled, by the recognition of the wonders of diversity and complexity that naturally evolving life can itself bring forth, and by both the limitations and ongoing potentials of our own humanity. It is often the case that people who understand and appreciate "all that" find that their "spirits" can be richly fulfilled in the largest sense through human science or art (and often through a combination of both), without experiencing the slightest need for a "spirituality" rooted in superstition and in the belief in gods and the churches of old. Far from being a recipe for a grey, cold and passionless outlook, a truly materialist scientific method, systematically applied to uncovering the actual genuine wonders of the natural and social world, can uncork the imagination, the sense of purpose and the transformative consciousness and initiative of human beings in ways no reference to a presumed higher power ever could. Isn't that an outlook worth striving for?


*It is not however the case, as Creationists often claim, that "all the major living groups" appeared at that time--in fact what we think of today as the major animal and plant "groups," including the reptiles, amphibians, birds, mammals, flowering plants and so on, didn't even begin to appear until millions of years later.

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**We know mitochondria and chloroplasts used to be separately reproducing independent organisms because every chloroplast or mitochondria to this day still contains its very own DNA, which is separate and different from the host cell's, and which gets reproduced "passively" every time the host cell reproduces.

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