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.
The answers proposed
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 ânice conditionsâ 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 âmisleadingâ 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
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.
The answers proposed
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 ânice conditionsâ 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 âmisleadingâ 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
Similar Books
