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Evolution’s ‘big bang’ explained (and it’s slower than predicted)

The sudden appearance of a range of modern animals about half a billion years ago, during evolution’s “big bang”, has intrigued and puzzled generations of biologists from Charles Darwin onwards. A new…

Marine life during the Cambrian explosion. A giant Anomalocaris investigates a trilobite, while Opabinia looks on from the right, and the ‘walking cactus’ Diania crawls underneath. Katrina Kenny & Nobumichi Tamura

The sudden appearance of a range of modern animals about half a billion years ago, during evolution’s “big bang”, has intrigued and puzzled generations of biologists from Charles Darwin onwards.

A new study by Greg Edgecombe from London’s Natural History Museum, Julien Soubrier from the University of Adelaide, and I, and published today in Current Biology, suggests that the evolution of all these animals during the lower Cambrian can be explained by only a relatively minor increase in evolutionary rates - sustained over 20-30 million years.

Minor changes acting cumulatively over substantial periods generate huge differences: Albert Einstein termed compound interest “the most powerful force in the universe”, and his observation also applies to evolutionary change.

The near-simultaneous appearance of a plethora of advanced animals around 530 million years ago is known as the Cambrian explosion.

Within a geologically brief interval of perhaps 20 million years, virtually every modern animal phylum made its fossil debut, including arthropods (represented today by insects, crustaceans and arachnids), molluscs (clams, snails, octopus and squid) and chordates (sea-squirts and vertebrates).

Alongside these familiar forms were a range of much more bizarre creatures, such as Opabinia (with five eyes and a stalked jaw), Diania (which looked like a walking cactus), and Anomalocaris (which looked like the head of a lobster grafted onto the body of a squid).

Five-eyed Opabinia. Credit: Nobumichi Tamura

Measuring evolution’s big bang

Evolution’s big bang has fascinated and perplexed scientists for hundreds of years. Some scientists have speculated (with little evidence) that animals evolved at light speed during this time, hundreds or thousands of times faster than they are evolving today.

It certainly made Charles Darwin feel uneasy: he thought incremental evolution through natural selection could not easily explain such an abrupt pattern.

Rather, he predicted that modern animal groups would appear in a staggered fashion, preceded by a range of precursors. Such reservations have predictably been exploited by opponents of evolution.

But it has been notoriously difficult to measure evolutionary rates during this pivotal interval of earth history, for good reason. = Such inferences require an exceptional fossil record.

Meet your flatmate. A living arthropod (centipede Cormocephalus) crawls over two 515-million-year-old relatives (Estaingia trilobites) which lived during the Cambrian explosion. All are found on what is now Kangaroo Island, southern Australia. Michael Lee

To observe and date changes in ancestral and descendant species, we need complete animals preserved in rocks that span a substantial and continuous interval of time. Unfortunately, the Cambrian fossil record is far too patchy: good fossils are thinly scattered across time and space, making it impossible to directly “read” evolutionary rates from the rocks.

Branches on the evolutionary tree

Our team used a novel approach to measure evolutionary rates during this pivotal moment in earth history, using living animals. Just as astronomers can infer much about the origin of the universe from the nature and movements of the galaxies today, we can learn much about the Cambrian explosion (the origin of animals) from the anatomy, genes, and current evolutionary trajectory of living animals.

We focused on arthropods, the dominant group of animals ever since the Cambrian, today making up more than 80% of animal life.

A fossilised trilobite (Olenoides serratus). Wikimedia Commons

We reconstructed a detailed evolutionary tree of arthropods and inferred how much change had occurred on every branch of this tree, purely by looking at living animals. Then, using fossils, DNA and an accommodating supercomputer, we worked out how fast each branch was evolving.

It turns out that – on average - Cambrian animals were evolving about five times faster than modern animals. This is fast, but nowhere near the speediest estimates that had been previously thrown about.

More importantly, such rates are totally consistent with Darwinian evolution by natural selection: for instance, mammals that colonise novel island habitats evolve a few times faster than their mainland relatives.

What fuelled the explosion?

What could have driven prolonged rapid (but not impossibly fast) evolution during evolution’s big bang? Many game-changing adaptations first appeared during the Cambrian explosion, such as vision, predation, burrowing, and active swimming.

These innovations would also have opened up totally new niches that animals would have raced to exploit, and would have triggered rapid evolutionary “arms races” between predators and prey.

The magnitude of these innovations meant their evolutionary effects could have reverberated for substantial time. While the Cambrian explosion occurs over a time interval that is considered short on geological scales, it still occupied a considerable period.

The inferred five-fold increase in evolutionary rates, acting over perhaps 20 million years, would generate 100 million years “worth” of evolutionary change.

That’s a huge amount of evolution: for instance, whales and bats both diverged from a tiny shrew-like common ancestor over perhaps as few as 65 million years.

This helps resolves Darwin’s dilemma: moderately elevated evolutionary rates - sustained over 20 million years in the early Cambrian - could easily explain the relatively sudden appearance of a range of highly divergent modern animals.

Join the conversation

15 Comments sorted by

  1. David Arthur

    resistance gnome

    What's the basis for the ideology that evolution must proceed at a constant rate, anyway?

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    1. Dale Bloom

      Analyst

      In reply to David Arthur

      I can remember being roused on by an evolutionary biologist for mentioning the words “punctuated equilibrium”

      If it had been a classroom, I may have been told to write out the word “Darwin” 1,000 times.

      But this might help explain the furore, brawling and divisiveness often occurring amongst evolutionary biologists.

      “In punctuated equilibrium, species tend to show morphological stasis between abrupt speciation events, whereas in phyletic gradualism species undergo more continuous change”

      http://www.sersc.org/journals/IJBSBT/vol3_no4/3.pdf

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    2. David Arthur

      resistance gnome

      In reply to Dale Bloom

      What I find amusing is the notion that, even if DNA mutates at a constant rate (which assumes that the potential surface for all possible DNA configurations is dead flat, an amusing enough notion in itself), that this should produce a constant rate of morphological change.

      "Darwin" a thousand times? Is that a thousand bottles of NT draught?

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    3. Michael Shand

      Software Tester

      In reply to Dale Bloom

      I very much doubt any decent teacher would have scoulded you for mentioning punctuated equilibrium

      Sounds like something a creationist might say, you know the old "Evolution is a religion and anyone who dares defy it is cast out of academia" argument

      similar to the "Climate Change is a religion" argument

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    4. Dale Bloom

      Analyst

      In reply to Michael Shand

      At the time, if someone mentioned the words “punctuated equilibrium” then they were challenging the very notion of Darwinism, which inferred that evolution was occurring at a constant rate.

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    5. Don Gibbons

      Clerk

      In reply to Dale Bloom

      Hi Dale. I don't know which evolutionary biologist rounded on you for referring to punctuated equilibrium. Maybe they were strict Dawkinists. Stephen Jay Gould and Niles Eldridge, who coined the term "punctuated equilibrium" never challenged the notion of Darwinism, if by "Darwinism" you mean evolution by natural selection. Tempo and mode of evolution by natural selection has been a source of contention amongst some workers, but I am not sure that "punk eke" and gradualism are mutually exclusive.

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    6. Dale Bloom

      Analyst

      In reply to Don Gibbons

      Try mentioning the words “irreducible complexity”, and certain evolutionary biologist teachers may make you write out the word “Darwin” 100,000 times.

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    7. Philip Impey

      Architect+Urban Designer

      In reply to David Arthur

      If this latest discovery can't fit in with Darwin's theory of evolution, why try to again add another proviso to his theory to attempt to "shoe-horn" the evidence into fitting what appears ever increasingly to be a dogmatic belief system. Isn't it about time we put aside Darwinian thought and look at origins through a new lens?

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    8. David Arthur

      resistance gnome

      In reply to Philip Impey

      To my knowledge, Darwin's great insight was into Selection among variants. He knew nothing of the mechanism of how that variation arises.

      If you think and enquire about it, you'll find that biology and hence science has incorporated Darwinism, and undergone further development since then.

      I recommend Brian Goodwin's "How the Leopard Changed Its Spots".

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    9. Don Gibbons

      Clerk

      In reply to Philip Impey

      Hi Philip,
      What "new lens" did you have in mind? I think the article above adressed your concerns. Nowhere does it say that this latest research is inconsistent with evolution by natural selection;
      "This helps resolves Darwin’s dilemma: moderately elevated evolutionary rates – sustained over 20 million years in the early Cambrian – could easily explain the relatively sudden appearance of a range of highly divergent modern animals."
      Have a look at Ediacaran and Tommotian faunas. How many decamillions of years do you need to to define an evolutionary process as "gradual"? No shoe-horn required.

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  2. John Newlands

    tree changer

    500m years ago there was an explosion in the number of low metabolic rate species. 100 years ago there was 7-fold increase in the population of a single species whose extended energy needs included cars and electricity, not the pickings from seafloor mud. Observers from another galaxy might find the h. saps explosion more puzzling.

    Like some of the critters depicted we might find ourselves maladapted, particularly to weather extremes and lack of external energy input I've been on some trilobite digging trips and the niggling thought is 'why didn't they survive, are we next?'.

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  3. George Michaelson

    Person

    Great article. Don't entirely understand the molecular analysis opportunities in the burgess stuff, hasn't all the biological material been replaced by shale forming deposits?

    Pushing back from the model looks really cool. I see you did a heap of grid computing on the paper, again I'm really stoked how this stuff helps inform biology outcomes (I used to work in university CS so relate to this kind of big box service more than the science carried out on it)

    I believe Arthropods occupy pretty specific niches. they have tight bindings to the food opportunities they are presented to. Is this the kind of external/environmental pressure which is the forcing function of the rate of change?

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  4. Robert Tony Brklje
    Robert Tony Brklje is a Friend of The Conversation.

    retired

    You have to be careful when studying fossils to remember that the deposition and preservation of those fossils requires specific and more unusual than usual circumstance.
    So environmental conditions that favour the creation of fossils versus the complete recognisable loss of the deceased life form. With regard to accessible regions for fossil investigation in today's environment.
    Taking that into account a evolution big bang, might appear but it also could just appear to be real as a result of the huge amount of yet to be discovered or totally lost fossil record.
    The majority of the earth's surface for the majority of the time does not present the required circumstance to create and preserve fossils and at this time we can only gain access to a minor fraction of that with regard to what is environmentally accessible at this time.

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  5. Paul Prociv

    ex medical academic; botanical engineer at University of Queensland

    Fascinating stuff. As "random mutations" underlie evolution, and cosmic radiation influences rates of mutation, surely hypotheses have been proposed about this as a possible major influence at that time. No doubt, the atmosphere would have been dramatically different then from now, with much lower oxygen levels, and therefore far less ozone up high, so that animals would have been exposed to much more radiation - but only those on the surface, or in shallow waters, I suppose . . .

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