A Biological Big Bang

The Cambrian Explosion Becomes More Explosive

New high-resolution age data narrows the date range for the paleon-tological event known as the Ediacaran-Cambrian transition to 410,000 years. A period of 410,000 years seems a long time to most people, but to scientists who study Earth's history and life, it's a mere moment. Discovery of a novel life-form that appeared within such a "moment" in the earliest days of the Cambrian era carries profound implications. When viewed in light of the measured "molecular clock" rate, this new finding presents a serious challenge to the commonly accepted story of life's slow, gradual development by natural processes alone.

Life's Explosion onto the Scene

The Cambrian "explosion" refers to the sudden (geologically speaking) and virtually simultaneous appearance of a wide diversity of phyla—the largest number known to have existed in the 3.8-billion-year record of life.¹ As you may recall from your high-school biology class, a phylum is a category of life-forms that share the same basic body plan. In the taxonomic classification system set forth by botanist Carl Linnaeus, a phylum ranks just below a "kingdom" in terms of its breadth, or inclusiveness.

The Cambrian explosion, more than a half-billion years ago, saw the rapid appearance of mostly marine phyla, the first creatures to possess skeletons, digestive tracts, circulatory systems, and complex internal and external organs. They appeared very soon after sufficient oxygen became available in Earth's atmosphere and oceans to make their existence possible.

Even the phylum in which humans belong, the chordate phylum, appeared in the Cambrian period. The chordate phylum is characterized by the presence of a dorsal hollow nerve cord and a notochord. All vertebrates and even some invertebrates belong to the chordate phylum. Paleontologists had already been surprised to discover fossils of chordates, including some vertebrates, dating back to the beginning of the Cambrian explosion.

Dating the Beginning of the Cambrian Explosion

Two of the best attempts to determine a firm date for the ignition of the Cambrian explosion were undertaken by teams led by Diazhao Chen and Can Chen, respectively. Diazhao Chen and four colleagues used the uranium-lead method to determine the ages of zircons found in two adjacent strata of the Liuchapo Formation in South China. One of the two strata contained fossils of (simpler) life from the Ediacaran era (just previous to the Cambrian), and the other contained fossils of Cambrian life. This juncture is referred to as the Ediacaran-Cambrian boundary.

The fossil record indicates that Ediacaran animals were the first to appear on Earth. They are characterized by tubular and frond-shaped filter feeders. The record also shows that Ediacaran fauna experienced a sudden mass extinction event. What followed almost immediately was the explosive appearance of vastly more complex Cambrian animals.

Diazhao Chen's team, using the uranium-lead method, determined that the base portion of the Liuchapo Formation dates back 542.1 ± 5.0 million years.² Using this same method, the mid-upper portion of this formation measured 542.6 ± 3.7 million years old.³ Five years later, Can Chen and his colleague Qinglai Feng refined this measurement (using a weighted-mean approach to measuring the uranium-lead zircon age) to demonstrate that the lower part of the Liuchapo Formation indeed marks the Ediacaran-Cambrian boundary.⁴ Their analysis reveals the boundary date to be 540.7 ± 3.8 [±6.6] million years ago. (The unbracketed error bar is the probable statistical error. The bracketed error bar represents the probable systematic error.)

A More Precise Date

Ulf Linnemann and a team of fourteen geochemists provided an even more precise date for the end of the Ediacaran and the launch of the Cambrian. The team found a composite geological section of the Ediacaran-Cambrian boundary in southern Namibia. This site allowed for biostratigraphic as well as chemostratigraphic analysis, bracketed by radiometric dating.⁵ Their measurements constrained the date for the Ediacaran-Cambrian boundary to no earlier than 538.99 ± 0.21 million years ago and no later than 538.58 ± 0.19 million years ago.

The Linnemann team's date indicates that the Ediacaran-Cambrian boundary is 2 million years more recent than the dates proposed by Diazhao Chen's team and by Can Chen and Qinglai Feng. Though different, it is considered consistent, not discrepant, in that the three dates all agree within the error bars of each. What their findings more notably reveal is the brevity, or narrowness, of that boundary. According to the Linnemann team's measurements, the faunal transition from Ediacaran to Cambrian biota appears to have occurred within a time frame of a mere 410,000 years.⁶

Another Phylum Discovered

Research that followed Linnemann's team's discovery unearthed yet another breakthrough: evidence of a still-surviving phylum that no one previously detected in Cambrian strata. These bryozoans, commonly known as moss animals, are tiny, soft-bodied aquatic invertebrates, with body sizes ranging from 0.1 to 1.0 millimeters across. They are filter-feeders, equipped with tiny tentacles.

Paleontologists once thought the bryozoa phylum originated during the Tremadocian Stage, 485–478 million years ago. Though fossils of such minuscule, soft-bodied creatures living as long as 485 million years ago would be extraordinarily difficult to find, biologist Zhiliang Zhang nevertheless led an international team of ten geologists and biologists in an ardent search for them. This team painstakingly scoured Cambrian strata in South China and Australia. A recent issue of Nature reports their astounding success, the discovery of bryozoa fossils in the early Cambrian layer.⁷ Thus, yet another phylum has been added to the list of phyla now known to have emerged near the launch of the Cambrian explosion.

The mid-Cambrian, 520–535 million years ago, is where paleontologists have discovered the greatest number and diversity of Cambrian species. It is this epoch that has the richest, most extensive fossil beds. Increasingly, however, paleontologists are discovering phyla they thought originated in the mid-Cambrian actually being found in the early Cambrian deposits.⁸

These discoveries demonstrate that during the Cambrian period the diversification of higher taxa occurs before that in lower taxa. As paleontologists Douglas Erwin, James Valentine, and John Sepkoski have observed, "The major pulse of diversification of phyla occurs before that of classes, classes before that of orders, and orders before that of families."⁹

Philosophical Consequences

The transition from Ediacaran to Cambrian phyla in less than 410,000 years challenges all current and as yet conceivable naturalistic models for the evolution of life. Such a period would be far too brief for the extinction of one phylum and the appearance of a new one, let alone several. The differences between one phylum and another are not minor, and the observed processes by which current models propose that these changes occurred are too slow to account for them.

The known mechanisms of change for naturalistic evolutionary models—natural selection, mutations, gene exchange, and epigenetics—generate relatively small variations within individual organisms. Naturalistic models, therefore, predict that natural selection, mutations, gene exchange, and epigenetics will yield incremental variations within individual members of a species. Naturalistic models predict the diversification of species eventually will produce a new genus and with yet more time the diversification of genera will produce a new family. Diversification of families will produce a new order. Diversification of orders will produce a new class. Diversification of classes will produce a new phylum. The problem for naturalists, however, is that in the Cambrian period paleontologists observe the exact opposite of this sequence.

These recent findings appear to validate a comment by paleontologist Kevin Peterson in his review paper on the Cambrian explosion written more than a decade ago: "Elucidating the materialistic basis for the Cambrian explosion has become more elusive, not less, the more we know about the event itself."¹⁰

Evolutionary biologist Gregory Wray told his students and colleagues at Duke University in the early 1990s:

The Cambrian "explosion" of body plans is perhaps the single most striking feature of the metazoan fossil record. The rapidity with which phyla and classes appeared during the early Paleozoic coupled with much lower rates of appearance of higher taxa since, poses an outstanding problem for macroevolution.¹¹

The "problem" Wray references relates to all naturalistic evolutionary models. The obvious challenge for naturalistic models for life's history is to explain why the fossil record, especially the record for the Cambrian animals, shows the exact opposite of what the naturalistic models predict. New phyla appear first, not last. These phyla appear suddenly and without direct evolutionary precursors. Astronomers have noted that many Cambrian phyla appear the moment oxygen in Earth's atmosphere reaches the minimum level (10 percent) necessary for these phyla to survive.

These discoveries readily align with the notion that a Creator somehow intervened in the development of Earth's life. In fact, they match what would be expected from this creation perspective. This model posits that a divine Designer intervened to pack the planet with the diversity and abundance of animal life that would prove essential to humans in the launch of global civilization, an intention clearly expressed in the biblical creation texts. 

Notes
1. Precise estimates of the number of phyla that appear in the Cambrian period are found in Stephen C. Meyer, Darwin's Doubt: The Explosive Origin of Animal Life and the Case for Intelligent Design (HarperOne, 2013). See also Roger Lewin, "A Lopsided Look at Evolution," Science 241, no. 4863 (July 15, 1988), 291–293: doi:10.1126/science.241.4863.291.
2. Diazhao Chen et al., "New U-Pb Zircon Ages of the Ediacaran-Cambrian Boundary Strata in South China," Terra Nova 27, no. 1 (February 2015), 62–68: doi:10.1111/ter.12134.
3. Diazhao Chen et al., "New U-Pb Zircon Ages."
4. Can Chen and Qinglai Feng, "Carbonate Carbon Isotope Chemostratigraphy and U-Pb Zircon Geochronology of the Liuchapo Formation in South China: Constraints on the Ediacaran-Cambrian Boundary in Deep-Water Sequences," Palaeogeography, Palaeoclimatology, Palaeoecology 535 (December 2019): doi:10.1016/j.palaeo.2019.109361.
5. Ulf Linnemann et al., "New High-Resolution Age Data from the Ediacaran-Cambrian Boundary Indicate Rapid, Ecologically Driven Onset of the Cambrian Explosion," Terra Nova 31, no. 1 (February 2019), 49–58: doi:10.1111/ter.12368.
6. Linnemann et al., "New High-Resolution Age Data," 49.
7. Zhiliang Zhang et al., "Fossil Evidence Unveils an Early Cambrian Origin for Bryozoa," Nature 599 (Nov. 11, 2021), 251–255: doi:10.1038/s41586-021-04033-w.
8. M. Gabriela Mangano and Luis A. Buatois, "The Rise and Early Evolution of Animals: Where Do We Stand from a Trace Fossil Perspective?" Royal Society: Interface Focus 10, issue 4 (June 12, 2020): doi:10.1098/rsfs.2019.0103; Fangchen Zhao et al., "Complexity and Diversity of Eyes in Early Cambrian Ecosystems," Scientific Reports 3 (Sept. 25, 2013): doi:1o.1038/srep02751; Timothy P. Topper et al., "A Stem Group Echinoderm from the Basal Cambrian of China and the Origins of Ambulacraria," Nature Communications 10 (March 25, 2019): doi:10.1038/s41467-019-09059-3; Russell D. C. Bicknell and John R. Paterson, "Reappraising the Early Evidence of Durophagy and Drilling Predation in the Fossil Record: Implications for Escalation and the Cambrian Explosion," Biological Reviews: Cambridge Philosophical Society 93, issue 2 (May 2018), 754–784: doi:10.1111/brv.12365.
9. Douglas H. Erwin, James W. Valentine, and J. John Sepkoski Jr., "A Comparative Study of Diversification Events: The Early Paleozoic Versus the Mesozoic," Evolution 41, issue 6 (Nov. 1, 1987), 1183: doi:10.1111/j.1558-5646.1987.tb02459-x.
10. Kevin J. Peterson, Michael R. Dietrich, and Mark A. McPeek, "MicroRNAs and Metazoan Macroevolution: Insights into Canalization, Complexity, and the Cambrian Explosion," BioEssays 31, no. 7 (July 2009), 737: doi:10.1002/bies.200900033.
11. Gregory A. Wray, "Rates of Evolution in Developmental Processes," American Zoologist 32, no. 1 (February 1992), 131: jstor.org/stable/3883743.