Complex life forms appear suddenly in the Cambrian explosion, with no
Morris, Henry M. 1985. Scientific Creationism. Green Forest, AR: Master
Watchtower Bible and Tract Society. 1985. Life--How Did It Get
Here? Brooklyn, NY, pp. 60-62.
- The Cambrian explosion was the seemingly sudden appearance of a
variety of complex animals about 540 million years ago (Mya), but it
was not the origin of complex life. Evidence of multicellular life
from about 590 and 560 Mya appears in the Doushantuo Formation in
China (Chen et al. 2000, 2004), and diverse fossil forms occurred
before 555 Mya (Martin et al. 2000). (The Cambrian began 543 Mya.,
and the Cambrian explosion is considered by many to start with the
first trilobites, about 530 Mya.) Testate amoebae are known from
about 750 Mya (Porter and Knoll 2000). There are tracelike fossils
more than 1,200 Mya in the Stirling Range Formation of Australia
(Rasmussen et al. 2002). Eukaryotes (which have relatively complex
cells) may have arisen 2,700 Mya, according to fossil chemical
evidence (Brocks et al. 1999). Stromatolites show evidence of
microbial life 3,430 Mya (Allwood et al. 2006). Fossil
microorganisms may have been found from 3,465 Mya (Schopf 1993). There
is isotopic evidence of sulfur-reducing bacteria from 3,470 Mya (Shen
et al. 2001) and possible evidence of microbial etching of volcanic
glass from 3,480 Mya (Furnes et al. 2004).
- There are transitional fossils within the Cambrian explosion fossils.
For example, there are lobopods (basically worms with legs) which are
intermediate between arthropods and worms (Conway Morris 1998).
- Only some phyla appear in the Cambrian explosion. In particular, all
plants postdate the Cambrian, and flowering plants, by far the
dominant form of land life today, only appeared about 140 Mya (Brown
Even among animals, not all types appear in the Cambrian. Cnidarians,
sponges, and probably other phyla appeared before the Cambrian.
Molecular evidence shows that at least six animal phyla are Precambrian
(Wang et al. 1999). Bryozoans appear first in the Ordovician. Many
other soft-bodied phyla do not appear in the fossil record until much
later. Although many new animal forms appeared during the Cambrian,
not all did. According to one reference (Collins 1994), eleven of
thirty-two metazoan phyla appear during the Cambrian, one appears
Precambrian, eight after the Cambrian, and twelve have no fossil
And that just considers phyla. Almost none of the animal groups that
people think of as groups, such as mammals, reptiles, birds, insects,
and spiders, appeared in the Cambrian. The fish that appeared in the
Cambrian was unlike any fish alive today.
- The length of the Cambrian explosion is ambiguous and uncertain, but
five to ten million years is a reasonable estimate; some say the
explosion spans forty million years or more, starting about 553 million
years ago. Even the shortest estimate of five million years is hardly
- There are some plausible explanations for why diversification may have
been relatively sudden:
- The evolution of active predators in the late Precambrian likely
spurred the coevolution of hard parts on other animals. These hard
parts fossilize much more easily than the previous soft-bodied
animals, leading to many more fossils but not necessarily more
- Early complex animals may have been nearly microscopic. Apparent
fossil animals smaller than 0.2 mm have been found in the Doushantuo
Formation, China, forty to fifty-five million years before the
Cambrian (Chen et al. 2004). Much of the early evolution could have
simply been too small to see.
- The earth was just coming out of a global ice age at the beginning of
the Cambrian (Hoffman 1998; Kerr 2000). A "snowball earth" before
the Cambrian explosion may have hindered development of complexity or
kept populations down so that fossils would be too rare to expect to
find today. The more favorable environment after the snowball earth
would have opened new niches for life to evolve into.
- Hox genes, which control much of an animal's basic body plan, were
likely first evolving around that time. Development of these genes
might have just then allowed the raw materials for body plans to
diversify (Carroll 1997).
- Atmospheric oxygen may have increased at the start of the Cambrian
(Canfield and Teske 1996; Logan et al. 1995; Thomas 1997).
- Planktonic grazers began producing fecal pellets that fell to the
bottom of the ocean rapidly, profoundly changing the ocean state,
especially its oxygenation (Logan et al. 1995).
- Unusual amounts of phosphate were deposited in shallow seas at the
start of the Cambrian (Cook and Shergold 1986; Lipps and Signor
- Cambrian life was still unlike almost everything alive today.
Although several phyla appear to have diverged in the Early Cambrian
or before, most of the phylum-level body plans appear in the fossil
record much later (Budd and Jensen 2000). Using number of cell types
as a measure of complexity, we see that complexity has been
increasing more or less constantly since the beginning of the
Cambrian (Valentine et al. 1994).
- Major radiations of life forms have occurred at other times, too. One
of the most extensive diversifications of life occurred in the
Ordovician, for example (Miller 1997).
- Allwood, A. C. et al. 2006. Stromatolite reef from the Early
Archaean era of Australia. Nature 441: 714-718. See also Awramik,
Stanley M. 2006. Respect for stromatolites. Nature 441: 700-701.
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molecular fossils and the early rise of eukaryotes. Science 285:
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early life. Science 285: 1025-1026.
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plants. Science 285: 990-991.
- Budd, Graham E. and Sören Jensen. 2000. A critical reappraisal
of the fossil record of the bilaterian phyla. Biological Reviews
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atmospheric oxygen concentration inferred from phylogenetic and
sulphur-isotope studies. Nature 382: 127-132. See also: Knoll, A.
H., 1996. Breathing room for early animals. Nature 382: 111-112.
- Carroll, Robert L., 1997. Patterns and Processes of Vertebrate
Evolution. Cambridge University Press.
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phosphatized embryos from the Doushantuo Formation of China. Proceedings
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97(9): 4457-4462. http://www.pnas.org/cgi/content/full/97/9/4457
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million years before the Cambrian. Science 305: 218-222,
. See also
Stokstad, E., 2004. Controversial fossil could shed light on early
animals' blueprint. Science 304: 1425.
- Collins, Allen G., 1994. Metazoa: Fossil record.
- Conway Morris, Simon, 1998. The Crucible of Creation, Oxford.
- Cook, P. J. and J. H. Shergold (eds.), 1986. Phosphate Deposits of
the World, Volume 1. Proterozoic and Cambrian Phosphorites. Cambridge
- Furnes, H., N. R. Banerjee, K. Muehlenbachs, H. Staudigel and M. de
Wit, 2004. Early life recorded in Archean pillow lavas. Science
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Science 281: 1342-1346. See also: Kerr, Richard A., 1998. Did an
ancient deep freeze nearly doom life? Science 281: 1259,1261.
- Kerr, Richard A., 2000. An appealing snowball earth that's still hard
to swallow. Science 287: 1734-1736.
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Terminal Proterozoic reorganization of biogeochemical cycles.
376: 53-56. See also Walter, M., 1995. Faecal pellets in world
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created 2001-3-31, modified 2006-12-5