253

Geomagnetic Field Effects on Living Systems

6.3.7 The Cambrian Explosion of Life on Earth

In contrast to the mass extinction, Doglioni et al. (2016) proposed the hypothesis that “the Cambrian

explosion of life on Earth” could have been enabled by the increase of the GMF intensity mainly leading

to shield UV radiation. Marine prokaryotic organisms dominated the Earth for nearly 3 billion years

since they were protected by water from UV radiation (Doglioni et al., 2016). Some microbes are the

organisms that are most resistant to UV radiation (Doglioni et al., 2016).

Te Cambrian explosion was a great fowering of life forms that occurred between roughly 542 and 530

Ma; during this period practically all major animal phyla (body plan), and many now-extinct ones were

established. Highly diferentiated multicellular animals such as various corals, shellfsh, brachiopods,

and trilobites are found in the Cambrian strata, but few animal fossils are found in the strata before the

Cambrian period. In 1998, evolutionary biologist and paleontologist Andrew R. Parker of the Australian

Museum in Sydney at that time proposed the “light switch theory” as the cause of the Cambrian explo­

sion, which increases the selection pressure due to the emergence of eye-catching organisms (Parker,

1998). For the frst time in the history of life on Earth, trilobites, organisms with eyes, were born, and by

actively preying on others, it became advantageous for organisms without eyes (Parker, 1998). Te theory

is that organisms having acquired eyes and hard tissues are more able to adapt for defense against their

predation. As a result, fossil records appear to have exploded in a short period of time (Parker, 1998). He

estimates that the Cambrian explosion is a phenomenon in which many phyla acquired hard tissue all at

once at the same time (Parker, 1998). An important criticism remained in the timing of the relationship

between the rise of predation and the period when vision developed. In Parker’s review of many of the

debates about the transformation of life forms around the Precambrian–Cambrian boundary, Martin

(2009) argues that predation played a critical role during the Cambrian explosion, but that Parker’s

emphasis on vision is misplaced because eyes did not develop until late in the spread of such predation.

It is conceivable that the cause of the Cambrian explosion could be related to the termination of the

“snowball Earth,” which is global glaciation that the whole Earth was frozen during the most severe

ice age of Precambrian time. Here, the term “snowball Earth” was coined by a geobiologist, Joseph

Kirschvink of the California Institute of Technology (Caltech), in a short paper published in 1992 within

a lengthy volume concerning the biology of the Proterozoic Eon (Kirschvink, 1992). Te major contribu­

tions from his insight were: (1) the recognition that the presence of banded iron formations is consistent

with such a global glacial episode, and (2) the introduction of a mechanism by which to escape from a

completely ice-covered Earth, specifcally, the accumulation of CO2 from volcanic outgassing leading

to an ultra-greenhouse efect (Kirschvink, 1992). Moreover, Hofman et al. (1998) suggested that the

snowball Earth ended abruptly when subaerial volcanic outgassing raised atmospheric CO2 to about 350

times the modern level. Te rapid termination would have resulted in a warming of the snowball Earth

to extreme greenhouse conditions (Hofman et al., 1998). Te CO2 transfer to the ocean would result in

the rapid precipitation of calcium carbonate in warm surface waters, producing the cap carbonate rocks

observed globally (Hofman et al., 1998). However, it was supposed that it would be an indirect relation­

ship, if any, because the Cambrian explosion started at least 32 Myr afer the end of the snowball Earth

(Marshall, 2006). Moreover, the cold periods of the snowball Earth may even have delayed the evolution

of large size organisms (Bengtson, 2002).

Te Ediacaran gave way to the Cambrian explosion in 542 Ma afer the Kotlinian Crisis (~550–542

Ma). New species with complex body plans, hard parts for defense, and sophisticated eyes have emerged

rapidly. Burrowing also became more common and diverse, destroying the once widespread bacterial

mats and sending O2 to the seafoor, creating a new livable space. Tere are many factors that may

explain why the Cambrian explosion occurred, but the researchers’ idea of “fight from light” adds a

novel possibility to the debate. David Harper, a paleontologist at Durham University in the United

Kingdom who was not involved in the study, says that the researchers have opened up yet another excit­

ing and imaginative area of research within which to frame and test new hypotheses for the origin and

early evolution of animal-based communities (Randall, 2016).