A New History of Life (20 page)

Another animal found in abundance in the oldest of the Cambrian-aged deposits are sponges. Like the cnidarians, sponges show no respiratory structures, nor would we expect any. With a body plan built around a series of sacs (like the cnidarians, but with even less organization: there are no true tissues in a sponge), all sponges show a very high surface area to volume. In fact, sponges are like agglomerations of numerous single-celled organisms, with each cell essentially in contact with seawater. But even with this advantage, sponges show an even more efficient way of gaining oxygen. Their main feeding cell, called a choanocyte, causes large volumes of water to pass through the structure. Some sponge specialists have suggested that a sponge passes as much as ten thousand times its volume in seawater through its body each day. Consequently, sponges are capable of living in extremely low oxygen conditions because they so efficiently move large volumes of water through their body, getting enough oxygen even from water that has little.

The major groups of animals with hard parts in the Cambrian are obviously the huge tribe of arthropods, followed in numerical importance (in most Cambrian marine strata) by brachiopods, and then smaller numbers of echinoderms and mollusks. Brachiopods are a still-living group related to bryozoans that are routinely mistaken for bivalve mollusks. Yet while the shells of bivalves and brachiopods show a superficial similarity, the internal anatomy of the two groups are
radically different. The major feature of a brachiopod is a feeding organ known as a lophophore, composed of a large loop with numerous long, thin fingers producing a delicate fanlike shape within the shell. This organ filters seawater for food—and as it is filled with a body fluid, and is very thin, it serves also as an exquisite respiratory organ. For some of us, the brachiopods are a tragic group. Perhaps the most common inhabitants of Paleozoic sea bottoms, they were nearly wiped out by the Permian extinction ~250 million year ago, and never regained dominance.

Cambrian echinoderms make up a weird assemblage of small boxlike animals. Among the earliest echinoderms were peculiar, pinecone-shaped helioplacoids, with some primitive stalked eocrinoids and edrioasteroids found in some deposits as well. More common than echinoderms were mollusks. Most during the Cambrian were small in size, and each of the major classes (gastropods, bivalves, and cephalopods) is found in Cambrian strata. The most common mollusks, however, were monoplacophorans, a minor class today, but common in the Cambrian. They had a limpet-like shell and a snail-like body with a broad, creeping foot. Most interestingly, alone among mollusks of the time they showed a body organization that suggests segmentation. From looking at muscle scars on the fossil shells and comparing anatomy from the still-living forms, we think the Cambrian monoplacophorans had multiple gills. Modern-day gastropods have a single pair of gills, or even just a single gill. But the Cambrian monoplacophorans, which lived a very snail-like existence in all likelihood, found it necessary to have multiple gills. They are celebrated as the ancestral mollusk that would give rise to all the rest: the gastropods, cephalopods, bivalves, chitons, and more minor molluscan classes.

Long thought to have gone extinct at the end of the Permian, the discovery of living monoplacophorans in deep-sea settings in the 1950s led to a much greater understanding of the life of the early mollusks. The living forms confirmed what muscle scars found on the interior of the earliest monoplacophoran fossils asserted—that there was more than a single pair of gills. In fact, multiple pairs of muscles line the entire length of the interior of the shell, leading to the conclusion that
these early forms showed an evident segmentation or at least repeat of the gill–blood vessel system. Since it is only the gills (and supporting blood and filtering systems) that show this repeated pattern, it can be surmised that as in arthropods, this repeated pattern was an adaptation for increased respiratory surface area of the gills. A somewhat similar pattern of repetition, extending even to the shell, is found in the chitons, today commonly found on intertidal beaches.

Like the body of an echinoderm, the interior of a brachiopod shell is almost all water. There is very little flesh, and what is there stays in contact with a steady flow of seawater. The brachiopod lophophore creates several currents of seawater that pass into the sides of the shell, move across the lophophore, and are then sent out the front of the shell. This constant stream of new water entering a brachiopod has the same effect as the current passing through a sponge. The small volume of flesh to the great surface area of the lophophore, coupled with the steady flow of water (many times the volume of the interior of the shell), makes the brachiopod consummately adapted for a world of low oxygen.

Earlier in this book we noted the advance of entirely new disciplines of science, most notably astrobiology and its allied field, geobiology. But another field, this one a traditional mainstay of the biological sciences, mainly evolutionary development, has undergone a renaissance so important that it can almost be considered a new field as well. Its practitioners now call it evo-devo, and breakthroughs in this field have had a lot to say about the Cambrian explosion in the last decade. One of the greatest of evo-devo practitioners, Sean Carroll, has given us an exquisite tour of this newly revivified area of science in his 2005 book
Endless Forms Most Beautiful
.
¹⁴
If there is any single theme in this work, it is that science can now understand far better one of the previously intractable problems in evolutionary biology: the origin of novelty. How evolutionary innovation took place over relatively short periods of time just could not be explained by traditional Darwinian
concepts of evolution. The radical breakthroughs—be it the appearance of wings, legs for land, segmentation in arthropods, or even large size, the hallmark of the Cambrian explosion—could not stand up to stories about many and sudden mutations all working in concert to somehow radically change an organism. Evo-devo now seems to have solved this, and in his book, Carroll lists four aspects that combined can explain sudden evolutionary innovation that nicely encapsulates the new way of explaining how radical changes did take place.

The first “secret to innovation,” as Carroll puts it, is to “work with what is already present.” The concept that “nature works as a tinkerer” is central to this. Innovation does not always need a new set of equipment to build, or even a new set of tools. What is already present is the easiest route. Second and third are two aspects understood by Darwin himself: multifunctionality and redundancy.

Multifunctionality first is using an already present morphology or physiology to take over some second function in addition to that for which it was first evolved. Redundancy, on the other hand, is when some structure is composed of several parts that complete some function. If one of these can be then co-opted for some new kind of job, while the remaining parts are still able to function as before, there is in place a clear path for innovation that is far easier to use than the total de novo formation of some entirely novel morphology from scratch. Cephalopod swimming and respiration are like this. Cephalopods routinely pump huge quantities of water over their gills, and like many invertebrates used separated “tubes” or designated channels for water coming in and water being expelled, to ensure that oxygen-rich water is not rebreathed. But with minor morphological “tinkering” with this excurrent tube, a powerful new means of locomotion came about. Breathing and moving could now take place using the same amount of energy by utilizing the same volume of water for respiration and movement.

The final secret is modularity. Animals built of segments, such as the arthropods, and to a lesser extent we vertebrates, are already composed of modules. The limbs branching off arthropod segments have been amazingly modified into feeding, mating, and locomotion,
as well as many other functions. Arthropods are like a Swiss army knife, with each segment bearing limbs evolved to do a very specific function. The same is true in vertebrates with our digits, which have been modified to tasks as varied as walking on land to swimming to flying in the air. Not bad for some primitive fingers and toes! Where does the evo-devo come into play? It turns out that these morphologies are the soft putty for morphological change because they are underlain by systems of genetic “switches,” geographically located on the developing embryo in the same positions as the various limbs are found in the arthropod—or vertebrate.

Switches are the key here; they tell various parts of the body when and where to grow. One of the great discoveries is that the exact sequence of different body regions on an arthropod from its head to midregion to abdomen are lined up first on chromosomes in the same geographic pattern, and then on the developing embryo itself. Much of this is done by the crown jewels of the evo-devo kingdom: the Hox genes, and their differently named but equivalents in other taxonomic groups.

The many new discoveries of evo-devo have certainly been brought to bear on the many questions to be solved about that central mystery in the history of life, the Cambrian explosion, and the most important understandings of all: the timing of when and how the various animal phyla and thus separate body plans that we see today originated.

There have long been two schools of thought. The first is that the fossil record gives us a true picture of when the great differentiation of animals actually took place—phyletic divergence somewhere about 550 to perhaps 600 million years ago. But the second line of evidence comes from comparing genes of extant members of the ancient phyla, and using the concept of the “molecular clock,” mentioned earlier. At issue is when the most fundamental divisions in the animal kingdom take place—the split between an aggregate of phyla called protostomes and those called deuterostomes. These two groups are separated by fundamental anatomical and developmental differences in embryos.

The protostomes are composed of the arthropods, mollusks, and annelids among others, and they are characterized by embryos that as
they develop and grow following fertilization form a mouth out of a central opening in the growing larva called the blastopore. In deuterostomes (echinoderms, us vertebrates, and a number of minor phyla), the mouth and the blastopore remain separate. There is a third group, the very primitive phyla that split off from the main stem of animal evolution prior to the great protostome-deuterostomes split: these include the Cnidaria, sponges, and other jellyfish-like minor phyla.

The first to appear were the simplest forms, the cnidarians and sponges, which appear to be represented, as we have seen, in the Ediacaran assemblages of as much as 570 million years ago, the time interval before the Cambrian period (which began at 542 million years ago). But recognizable protostomes and deuterostomes are not seen until a short interval into the Cambrian period itself.

If the protostomes and deuterostomes split, what was the last animal before that split like? Many lines of evidence indicate that this creature was bilaterally symmetrical and was capable of locomotion. Many who ponder this time and its animals imagine this last common ancestor of both the protostomes and deuterostomes as a small featureless worm, perhaps like the modern-day
Planaria
, or the tiny and extant nematodes. But one of the great new discoveries is that this last member of the as yet undivided stock already had a genetic tool kit allowing it to begin some radical new engineering—and had such a tool kit for at least 50 million years before it was put into use! This worm would have had a mouth at front, anus at the rear, and a long tubelike digestive system in between. It may have had stubby projections sticking out of its side, perhaps for sensory information (touch and chemical sensing?). But the point is that all of this was set up in such a way that rapid transformation could—and did—take place. This is new. All the tools and features necessary for the Cambrian explosion sat around for 50 million years.

As noted above, the base of the Cambrian is dated now at 542 million years ago. The base of the period has been defined as the place in rock where the first identifiable locomotion marks are found in strata—a certain kind of trace fossil showing that animals, moving animals, were present and could make vertical burrows in the mud.
Yet for the next 15 million years, there seems to have been little formation of new body plans at all—or at least that we can find evidence of in the fossil record. The first real indication that a great diversification was taking place comes from the spectacular fossil beds only recently discovered in Chengjiang, China,
¹⁵
dated as 520 to 525 million years in age and mentioned above. It is an older version of the Burgess Shale in having common preservation of soft parts.

Both the Chengjiang and Burgess Shale faunas are dominated by arthropods—lots and lots of different kinds of arthropods. They soon became the most diverse animals on Earth—and have stayed that way ever since. There are some estimates that in our modern day, there may be as many as 30 million separate species of beetles alone!

Evo-devo tells us why. Of all the body plans, none can be so easily, quickly, and radically changed as arthropods. The reasons are just those listed above by Carroll: arthropods have modular parts, they have redundant morphologies that can be co-opted for new functions, and they have a series of Hox genes that allow ready transformation of specific regions in the overall body plan of segments throughout.

The old view has been that new animals mean that there must have been new genes coming into existence. There is sound logic in this. Surely a primitive sponge or jellyfish would have fewer genes than the more complex arthropods: it was argued that the common ancestor of all arthropod groups somehow added new genes—new Hox genes, as these are those that are the “switches” that tell the various parts of a body how to form and when. But such is not the case. Carroll and others showed that the last common ancestor of the arthropods did not evolve new genes; it already had them, and that the subsequent and amazing diversification of so many kinds of arthropods was done with existing genes. As Carroll put it: “The evolution of forms is not so much about what genes you have, but about how you used them.”