The Cambrian Explosion is often posed as a challenge for evolution because the sudden burst of change in the fossil record appears to be inconsistent with the more typical gradual pace of evolutionary change. However, although different in certain ways, there are other times of very rapid evolutionary change recorded in the fossil record -- often following times of major extinction. The Cambrian Explosion does present a number of challenging and important questions because it represents the time during which the main branches of the animal tree of life became established. It does not create a challenge to the fundamental correctness of the central thesis of evolution, the descent of all living species from a common ancestor. This important period in the history of life extended over millions of years, plenty of time for the evolution of these new body plans (phyla) to occur. Furthermore, the fossil record provides numerous examples of organisms that appear transitional between living phyla and their common ancestors. The ongoing research about the Cambrian period is an exciting opportunity to advance our understanding of how evolutionary processes work, and the environmental factors shaping them.

The major animal body plans that appeared in the Cambrian Explosion did not include the appearance of modern animal groups such as: starfish, crabs, insects, fish, lizards, birds and mammals. These animal groups all appeared at various times much later in the fossil record.³ The forms that appeared in the Cambrian Explosion were more primitive than these later groups, and many of them were soft-bodied organisms. However, they did include the basic features that define the major branches of the tree of life to which later life forms belong. For example, vertebrates are part of the Chordata group. The chordates are characterized by a nerve cord, gill pouches and a support rod called the notochord. In the Cambrian fauna, we first see fossils of soft-bodied creatures with these characteristics. However, the living groups of vertebrates appeared much later. It is also important to realize that many of the Cambrian organisms, although likely near the base of major branches of the tree of life, did not possess all of the defining characteristics of modern animal body plans. These defining characteristics appeared progressively over a much longer period of time.⁴

Interpretations of the “Cambrian Explosion”

Not all scientists accept the idea that the Cambrian Explosion represents an unusually rapid evolutionary transition. The fossil record is notoriously incomplete, particularly for small and soft-bodied forms. Some researchers argue that the apparent rapid diversification of body plans is an artifact of an increase in the rate of fossilization, due in part to the evolution of skeletons, which fossilize more effectively.⁵ Many of the early Cambrian animals possessed some type of hard mineralized structures (spines, spicules, plates, etc.). In many cases these, often very tiny, mineralized structures are all that are found as fossils. There were major changes in marine environments and chemistry from the late Precambrian into the Cambrian, and these also may have impacted the rise of mineralized skeletons among previously soft-bodied organisms. ⁶

Most scientists are persuaded that something significant happened at the dawn of the Cambrian era and view the Cambrian Explosion as an area of exciting and productive research. For example, scientists are now gaining a better understanding of what existed before the Cambrian Explosion as a result of new fossil discoveries. Recent discoveries are filling in the fossil record for the Precambrian fauna with soft-bodied organisms like those in the Ediacaran Assemblages found around the world.⁷ Late Precambrian fossil discoveries also now include representatives of sponges, cnidarians (the group that includes modern jellyfish, corals and anemones), mollusks and various wormlike groups. Some of the new fossil discoveries, in fact, appear to be more primitive precursors of the later Cambrian body plans. The discovery of such precursors shows that the Cambrian organisms did not appear from thin air.⁸ Further discoveries will no doubt reveal more clearly the relationship of Precambrian organisms with the creatures found in the Burgess Shale and Chengjiang deposits.⁹

Genomic studies provide further insights into the origins of the Cambrian Explosion. Although the genetic divergence of organisms would have preceded the recognition of new body plans in the fossil record, accumulating genomic data is broadly consistent with the fossil record.¹⁰ Both point to the rise of the bilateria (bilaterally symmetric invertebrate animals) in the latest Precambrian Ediacaran, and their ecological explosion in diversity in the Cambrian.

Unanswered Questions

The sudden change of the Cambrian Era was, in relative terms, not too sudden for the process of evolution. The changes during the Cambrian Era did not occur over decades, centuries, or even thousands of years; they occurred over millions of years—plenty of time for evolutionary change. However, for millions of years beforehand, body plans of animals had remained relatively constant. Not until this time period did a significant change occur. The remaining questions are: What triggered the Cambrian Explosion? And why did so much change occur at this time? Several different theories address the origin of the Cambrian Explosion, proposing that dramatic environmental changes must have opened up new niches for natural selection to operate upon. These proposals include the runaway glaciation theory,¹¹ which proposes that glaciers briefly covered much of the earth, and the resultant loss of habitat created bottlenecks where evolution could act more rapidly. Another theory suggests that a change in atmospheric oxygen led to this sudden burst in evolutionary changes.¹² Yet another proposal is that major changes in the seafloor, from algae mat-covered surfaces in the late Precambrian to soft muddy bottoms later in the Cambrian, had dramatic evolutionary and ecological impacts.¹³

The Cambrian Era Fossils, Providing Answers

While the causes of the Cambrian Explosion remain a topic of open and exciting debate, the continued fossil discoveries from the Cambrian and Precambrian Eras are bringing more clarity to the evolutionary puzzle. These fossils provide valuable insight, particularly for envisioning the common ancestors of diverse groups. For instance, both vertebrates (fish) and echinoderms (sea urchins, starfish) are part of the group called deuterostomes. Without fossil evidence, it is hard to envision what a common ancestor would look like for these very different creatures. The Cambrian fossils are filling in the picture.¹⁴

Organisms have changed significantly over time. In rocks more than 1 billion years old, only fossils of single-celled organisms are found. Moving to rocks that are about 550 million years old, fossils of simple, multicellular animals can be found. At 500 million years ago, ancient fish without jawbones surface; and at 400 million years ago, fish with jaws are found. Gradually, new animals appear: amphibians at 350 million years ago, reptiles at 300 million years ago, mammals at 230 million years ago, and birds at 150 million years ago.¹ As the rocks become more and more recent, the fossils look increasingly like the animals we observe today.

The Transition to Land: Sea Creatures to Land Animals

Fossils of land animals, or tetrapods, first appear in rocks that are about 370 million years old. In older rocks, only sea creatures are found. But in 1998, scientists found a fossilized fin, 370 million years old, with eight digits similar to the five fingers humans have on their hands, as shown in Figure 1. However, the fin was undoubtedly that of a fish, which means this fossil is strong evidence of a transitional form.

Figure 1:An Illustration of the fossilized fin found in 1998. Its resemblance to a tetrapod is an indication of gradual evolutionary change from sea creatures to land animals. Source: Image is used by permission from Falk, Coming to Peace, 113.

One of the great success stories in the examination of the fossil record was the finding of a near-perfect fossilized transition between a vertebrate adapted for water and one adapted for land. Evolutionary biologist Neal Shubin set out to find a more complete transitional specimen than the 1998 fin. He determined the exact age of rock that he expected would yield a transitional land/water animal, and then he and his team spent four summers in the Arctic scouring rocks of that age to find one. The results (see Figure 2 below) were spectacular.²

From Reptiles to Mammals

Mammals first appeared in the fossil record about 230 million years ago, nearly 70 million years after reptiles first appeared. One group of reptiles, the cynodonts, first appeared about 260 million years ago and became increasingly mammal-like in more recent fossils—circa 245 million years ago. This change can be seen most clearly in the bone structure of the ear, as illustrated in Figure 3.

Figure 3: As shown in the image above, transitional fossils of cynodonts had two jaw hinges. These fossils date from a time when the dentary and squamosal bones were beginning to take over the role of jaw hinge (hinge #2). This allowed the articular and quadrate bones to evolve into the second and third bones of the mammalian ear, as shown on the right. Source: Image used by permission from Falk, Coming to Peace, 119. Originally from F. H. Pough, J. B. Heiser, and W. N. McFarland, Vertebrate Life, 4th ed. (Upper Saddle River, NJ: Prentice Hall, 1996), 607.

Scientists found a species of cynodonts, dating to just before the emergence of mammals, that had a double jaw hinge like that of a mammal. A pair of bones found in even earlier cynodont fossils seems to have transitioned slowly into the ear. No other fossils have been found that share a similar structure to the transitional cynodonts and date back before the time of mammals. Likewise, soon after mammals appeared, these cynodonts became extinct. This timing implies that the cynodont fossils record the transition from reptiles to mammals.³

Transitional Forms: Few and Far Between

Transitional forms occur just when one might expect to see a change from one body type to another. However, a common objection is that few transitional fossils have been discovered; thus many lineages cannot be traced smoothly.

There are several reason for these gaps in the fossil record. First, fossilization is a very rare event. Plus, transitional species tend to appear in small populations, where rapid changes in the environment can provide a stronger evolutionary drive. Finally, because fossilization itself is a rare event, smaller populations are sure to produce fewer fossils. The fact that transitional species have been found at all is remarkable, and it offers further support of gradual, evolutionary change.