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The Explosive Appearance of Skeletal Designs

A new measure of the dramatic biological innovations that occurred during the Cambrian explosion has come to light in recent years.1 Based on fossils unearthed in southern China and in the Burgess Shale deposits of  the Canadian Rockies, biologists have discovered that 50 to 80 percent of the animal phyla (body plans) known to exist throughout Earth’s history appeared about 540 million years ago within a narrow window, a brief moment, of geologic time. Since then, few new animal phyla have appeared.

Along with signaling the “sudden” appearance of animal phyla, the Cambrian period marked the first time that animals with skeletons showed up.2 A skeleton may be (1) internal or external; (2) rigid or flexible; (3) formed with one, two, or multiple elements; (4) comprised of rods, plates, and solid three-dimensional parts; (5) grown by accretion, molting, or remodeling; and (6) composed of different chemical materials such as silica, calcium carbonate, calcium phosphate, and chitin.

Considering these and other characteristics, the team of researchers constructed a “skeletal space,” a mathematical space that defines all possible skeletal designs. Within this skeletal space, 182 possible skeletal designs were identified.

Interestingly, of these 182 possibilities, 146 appeared during the Cambrian event (a number based on analyses of 104 fossil genres recovered from the Burgess Shale site). In other words, according to the fossil record more than 80 percent of all possible skeletal designs appeared suddenly—during a period of less than 5 million years. Researchers consider the number of skeletal designs that pop up during the Cambrian explosion essentially maximal.

The Cambrian explosion has long represented a biological enigma. The more we learn about the introduction of complex animals on Earth, the more puzzling the Cambrian event becomes for evolutionary biologists. The explosive appearance in the Cambrian fauna of nearly all the possible skeletal designs defies a natural-process explanation. Yet, this is exactly what one would expect to see in the fossil record if the God of the Bible were responsible for the creation of animal life on Earth.

  1. R. D. K. Thomas, Rebecca M. Shearman, and Graham W. Stewart, “Evolutionary Exploitation of Design Options by the First Animals with Hard Skeletons,” Science 288 (2000):1239–42.
  2. Richard Cowen, History of Life, 3rd ed. (Malden, MA: Blackwell Science, 2000), 60–64.