Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 241

many biologists, including one leading neoDarwinian, generally admit to be often
erratic and unreliable because even within clades, rates of change are different for
different genes and vary over time ( Ayala, 1997; Valentine et al., 1999 ). It is no won-
der that the molecular clock estimates of the time of divergence of metazoan clades
by different authors have led to values that vary as widely as about 800 million
years, ranging from more than 1500 ( Bromham et al., 1998 ) to 670Mya ( Ayala
et al., 1998 ). So, for example, Runnegar (1986) estimated the time of divergence
of the animal phyla (protostome and deuterostomes) at 700 million years, Wray
et al. (1996) push it back to mid-Proterozoic, about 1200 Mya, which predates the
Cambrian explosion by 400-600 million years, while Wang et al. (1999) estimate
that it occurred about 1000 Mya.
The second insurmountable problem is that no fossil record or other empiri-
cal evidence corroborates or substantiates their estimates on the Proterozoic origin
of metazoans and protostome and deuterostome divergence. It is not easy to answer
why the bilaterians did not leave fossils, as they did later, if they existed at the time.
Deniers of the Cambrian explosion have conjectured that the Cambrian explosion
may be a paleontological artifact because soft-bodied, Pre-Cambrian metazoans that
evolved long before left no fossils and it was only during the Cambrian period that
they evolved exoskeletons and skeletons that made their fossilization possible. This
assumption is also rejected because most of the known Cambrian fossils are from
soft-bodied metazoans ( Conway Morris, 1998 ) and skeletization of metazoans is not
a universal feature of the Cambrian explosion. Besides, microscopic, nonmetazoan
fossils of soft-bodied Pre-Cambrian unicellulars abound. For the above reasons, the
molecular clock faces increasing criticism.
What to do with molecular data then? Keep in mind that Bromham et al. (1998)
had this advice:
Molecular dates can be misleading … . The molecular clock is unlikely ever to
replace the fossil record as the primary source of information on evolution in deep
time. But it has a critical role to play as an alternative historical narrative, poten-
tially complementing the biases and gaps of the paleontological record.
Bromham (2009)
The reality of the Cambrian explosion and its impact on the extant animal world
in our time is acknowledged by an overwhelming majority of biologists:
The Cambrian explosion is real and its consequences set in motion a sea-change in
evolutionary history.
Conway Morris (2000)
The Cambrian Explosion Conundrum
Although not fully understood, the “profound discontinuity” between the Cambrian
and Pre-Cambrian forms ( Conway Morris, 1989 ) is a well-known fact. The