In The Blink Of An Eye (8 page)

We now know of wonderfully preserved communities of animals where a diversity of animal phyla are represented from the Cambrian, but not before the Cambrian. As stated previously, the internal body plans of animal phyla evolved some 120 to more than 500 million years earlier (depending on who you believe). Hence, the variety of internal body plans found in animals today really was once hidden within the bodies of worms, for tens of millions of years. Now we can really understand
what
the Cambrian explosion is. It is the sudden acquisition, 543-538 million years ago, of hard external parts by all the animal phyla found today (except the sponges, comb jellies and cnidarians). It is the simultaneous transition from the prototype worm-shaped or soft-bodied form to complex, characteristic shapes (also known as ‘phenotypes') within each phylum, and it happened in a blink of an eye on the geological timescale. The
what
of the Cambrian explosion is now understood.

For some reason the early members of each animal phylum did not acquire their hard parts, and hence their characteristic external parts, until the Cambrian. This poses a different question - the
why
of the Cambrian explosion. Why did it take place? The evolution of hard, external parts was not a chance occurrence. It took place simultaneously in all phyla, after a considerable period during which nothing happened. This extensive correlation must have been forced by an external factor. But what factor? What caused the Cambrian explosion -
why did it happen
? This is the problem we are left with, and the aim of this book is to solve it.

Why did the characteristic external parts of animal phyla not evolve when the genetic identities were laid down in the Precambrian? Perhaps they simply did not need to. The development of complex, hard external parts from an embryo requires more energy than a simple sausage-shaped sac - why spend more energy than is necessary? And for some 120 million years or so they did not make the leap to external part development. The factor, then, that caused this leap, and made the expenditure of additional energy necessary, must have been monumental. In this book I aim to reveal the identity of this factor and hence the cause of the Cambrian explosion - the reason why it happened.

A number of explanations for the
why
of the Cambrian explosion have been put forward. Unfortunately, there is strong evidence against all of them: none can stand up to scientific scrutiny. The simplistic explanation is that the general environmental conditions were uniquely
befitting for evolution during the Cambrian. That is, this was simply a nice time and place for animals to evolve. This includes both the physical (non-living) and biological (living) factors within the environment. But recent finds of embryos of nonskeletised animals from the Cambrian have provided evidence against this rather circular argument. The eggs of two Cambrian animals, a jellyfish and a bristle worm, are large compared with those of their living ancestors. The considerable elbowroom within the egg, and the close resemblance of late embryos to their adult forms, are clues that Cambrian embryos hatched fully equipped to depart into the environment rather than passing through a series of less-than-proficient juvenile stages. This strategy, known as direct development, is common under harsh or unpredictable environmental conditions today. It ensures that offspring will survive rough times. For example, crabs usually hatch from their eggs as slow moving planktonic forms that drift around in the water. These young forms are easy prey for many fish and when times are hard even these meagre morsels become fish food. But if the young crabs hatch so that they can live on the sea floor, and possess colour pigments and shapes that blend into their backgrounds, they may escape the attention of predators and survive to become adults. This is not the usual method of development because a highly developed hatchling comes with a high energy cost to its parent. Direct development in the Cambrian is perhaps a surprise because it indicates that this period was not so hospitable after all. Out goes the ‘nice conditions' hypothesis.

Some other explanations of the cause of the Cambrian explosion have been victims of a general misunderstanding of
what
the Cambrian explosion really is. Many scientists have launched their research to expose the
why
armed with a very misleading explanation of this event - simply, the spontaneous evolution of all animal phyla. This is
not
a fair summary of the Cambrian explosion, and one which I will name the ‘misleading' explanation. Now we know that the Cambrian explosion was the spontaneous evolution of
external body parts
in all phyla, where the internal body plans of all phyla are already in place. To be fair, scientists in the past have misunderstood the Cambrian explosion through no fault of their own - the genetic evidence that tells the story of internal body plans is a recent finding.

Figure 1.8
Tow versions of the history of the animal phyla. From the first soft-bodied form, evolutionary branching is equivalent in both models. (A) indicates that both internal body plans and external parts diversified throughout this branching, and most theories on the cause of the Cambrian explosion have been based on this model. (B) is the correct model and properly identifies the Cambrian explosion - that it was the
simultaneous evolution
of external forms in all phyla.

Current evidence suggests that the Precambrian ‘event' - the evolution of internal body plans - was not explosive but gradual, lasting tens or hundreds of millions of years. This is likely because the Precambrian ‘event' concerned one animal form evolving from a previous form, and so on - a condition not affiliated with
the
Cambrian explosion. The Precambrian ‘event' was more a surge in evolution than an explosion. It is possible for
the
Cambrian explosion to happen at one moment in time, but not so the Precambrian ‘event'. To summarise, the old interpretation of the ‘Cambrian explosion' is actually the combination of
the
Cambrian explosion and the Precambrian ‘surge'. In general, the surge was the major genetic event and the explosion was rather more driven by some external factor.

The next proposals for the
why
of the Cambrian explosion suggested that the physical environmental conditions changed at the end of the Precambrian. We have already learnt that it was not a change in the environment as a whole (physical and biological factors) that caused the Cambrian explosion, but some other explanations centre on just one part of the environment. One explanation is based on a rise in atmospheric oxygen to a critical level, another on a decrease in atmospheric carbon dioxide. Oxygen and carbon dioxide are factors affecting the breathing and circulatory systems of animals. These systems are part of the internal body plans in most animal phyla, and generally affect external parts in a minor way. So oxygen and carbon dioxide levels could not have played a part in the Cambrian explosion; maybe they were involved in the evolution of internal body plans. Also, there is geological evidence indicating that oxygen levels peaked at various times before the Cambrian. Some of this evidence comes from cosmic spherules - small rocks that landed on Earth from outer space throughout geological history. Cosmic spherules contain chemicals that react with oxygen, and the degree of reactivity indicates the level of oxygen present in the Earth's atmosphere at the time of landing. And they reveal a series of peaks in oxygen levels, before, during and after the Cambrian.

Staying with the chemical theme, an additional physical environmental factor that may have changed during the Cambrian is the availability of phosphorus. Phosphorus facilitates the development of
calcium phosphate skeletons, and an increase in phosphorus levels could have led to an increased development of hard external parts. But it is not just calcium phosphate that makes up the external parts of animals - other chemicals are involved too. The phosphorus argument takes no account of these. And like oxygen, there is evidence that phosphorus levels peaked before
and
during the Cambrian.

Another physical environmental suggestion for the cause of the Cambrian explosion is that continental shelf areas (‘shallow' water habitats) increased at the beginning of the Cambrian. This condition may have been forged as seawater encroached on the land masses worldwide. But even if continental shelf areas increased, they were present to some degree long before the Cambrian explosion. Hence this event doesn't add anything new to the system, just more of the same.

The most recent physical environmental bid for an explanation of the
why
of the Cambrian explosion is linked to the ‘Snowball Earth' hypothesis. It is thought that before the Cambrian there were spells when the Earth looked like a giant snowball. In some Precambrian times, the sun was probably some 6 per cent fainter than it is today. The consequent drop in both temperature and concentrations of carbon dioxide in the atmosphere allowed polar ice caps to grow. Ice reflects sunlight and the infra-red radiation from the sun that heats the Earth's surface. So the more ice that formed, the cooler the planet became and the greater was the potential for further ice to form. A hardline view of this idea is that all the Earth's oceans eventually froze to a depth of about 1 kilometre. A softer view is one of greatly extended polar ice caps, leaving open water to circulate around the equator. In either case, normal conditions would resume after volcanic events filled the Earth's atmosphere with enough carbon dioxide to kick-start the greenhouse effect, or global warming, causing the ice to melt. These Snowball Earth events may have taken place regularly some 2,000 million years ago, but did so at least twice during the late Precambrian, between 850 and 590 million years ago. Inevitably, for an event taking place near to the Cambrian, Snowball Earth has been nominated as the cause of the Cambrian explosion. One problem is that the scientific jury has yet to decide on the hard or soft view of Snowball Earth. The soft version provides no explanation for the Cambrian explosion for the same
reason as the sudden increase in continental shelf hypothesis - namely there remained ‘normal' watery environments available to host evolution. But even the hard view is open to criticism as a cause of the Cambrian explosion.

Firstly, this idea has a teleological foundation. It assumes the course of evolution was predetermined from the beginning. We are given a situation where the Precambrian worm-like bodies of all animal phyla are just itching to take on their Cambrian forms, but ice puts everything on hold. Then, when the ice has gone, it is time for evolution again. This is not an objective view. As we have considered before, why should a convenient worm shape
have
to change? If the course of evolution was predetermined, why did it not continue in the water under the ice? The second major doubt cast over this laboured explanation for the
why
of the Cambrian explosion is that the figures simply do not balance. The Cambrian explosion took place between 543 and 538 million years ago. The last Snowball Earth event ended 575 million years ago at the latest. So there is a difference of at least 32 million years between these two events. This is fact. So a Precambrian Snowball Earth event cannot explain the
Cambrian
explosion, although it could have played a role in the Precambrian ‘surge'.

We are trying to explain an
explosion
in diversification, or a macro-evolutionary event. In terms of external parts, changes in physical environmental conditions lead only to micro-evolution, or gradual transitions. To explain the cause of an
explosion
we need a factor that is a matter of life and death. Such a factor must be part of the biological environment - a change took place in the animals themselves. And biological environmental explanations for the cause of the Cambrian explosion have also been proposed.