Page 210 - Introduction to Paleobiology and The Fossil Record

THE ORIGIN OF LIFE 197

(Fig. 8.11a). Slabs are sometimes covered with seaweed, based on its overall shape and, if this

great loops and coils of Grypania, preserved identification is correct, it is a eukaryote.
as thin carbonaceous fi lms. It has been identi- Many dispute this identifi cation, and would

fied as a photosynthetic alga, a type of argue that the oldest eukaryotes are micro-

Box 8.2 Dating origins

There was a sensation in 1996 when Greg Wray of Duke University and colleagues announced new

molecular evidence that animals had diversified about 1200 Ma. This estimate predated the oldest
animal fossils by about 600 myr. In other words, the molecular time scale seemed to be double the
fossil age. This proposal suggested three consequences: (i) the Precambrian fossil record of animals
(and presumably all other fossils) was even more defi cient than had been assumed; (ii) the Cambrian
explosion, normally dated at 542 Ma, would shift back deep into the Proterozoic; and (iii) all other
splitting dates in the UTL (see Fig. 8.4) would have to be pushed back deeper into the Proterozoic
and Archaean.
Wray’s view was confirmed by a number of other molecular analyses of basal animal groups, but

also of plants, Archaea and Bacteria. Their work is based on gene sequencing from RNA of the
nucleus, and it is calibrated against geological time using some fixed points based on known fossil

dates. The molecular clock model of molecular evolution (see p. 133) suggests that genes mutate at
predictable rates through geological time, so if one or more branching points in the tree can be fi xed
from known fossil dates, then the others may be calculated in proportion to the amount of gene
difference between any pair of taxa.
In Wray’s case, mainly vertebrate dates were used, the assumed dates of branching between different

groups of fishes and tetrapods in the Paleozoic. So, he had to extrapolate his dates from the Paleozoic

fixed points back into the Precambrian. Extrapolation (fixing dates outside the range) is tougher than

interpolation (fixing dates within a range between a known date and the present day): small errors on

those Paleozoic dates would magnify up to huge errors on the Precambrian estimates.
Wray’s calculations were criticized by Ayala et al. (1998), who recalculated a date of 670 Ma for
the basal radiation of animals, much more in line with the fossil record. In a further revision, Kevin
Peterson and colleagues from Dartmouth University (2004) showed that Wray had unwittingly found
a very ancient date because vertebrate molecular clocks tick more slowly than those of most other
animal groups. So, if vertebrate clocks are slower, it takes longer for a certain amount of genome
change to occur than in other animals, and so any calibrations extrapolated from such dates will be
much more ancient than they ought to be. Peterson et al. (2004) brought the date of divergence of
bilaterian animals down to 573–656 Ma, and so the split of all animals would be just a little older,
in line with Ayala et al.’s (1998) estimate.
The reconsideration of molecular clock methods has now opened the way for a great number of
studies of the dating of other parts of the UTL (see Fig. 8.4). Most analysts accept a baseline date
of 3.5–3.8 Ga for the universal common ancestor, the first living thing on Earth. For example, Hwan

Su Yoon and colleagues (2004) from the University of Iowa were able to reconstruct a tree of pho-
tosynthetic eukaryotes, the various algal groups, as well as plants (Fig. 8.10), and to date it. They
used fixed dates for the origin of life, the oldest bangiophyte red alga (see Box 8.3), the fi rst green

plants on land, the first seed plants, and higher branching points among gymnosperms and angio-
sperms (see pp. 498, 501). These then allowed the team to date splits among marine algae around
1.5 Ga, in line with fossil evidence, and a major radiation of photosynthetic eukaryotes from 1.0 Ga
onwards. Their dates also give information on the timing of some events in the endosymbiotic model
for the acquisition of organelles by green plant cells (Fig. 8.10).
Read more about the three-domain tree of life at http://www.blackwellpublishing.com/
paleobiology/.

Continued