Page 214 - Introduction to Paleobiology and The Fossil Record

THE ORIGIN OF LIFE 201

250 μm

25 μm

(a) (b)

Figure 8.13 The oldest multicellular eukaryote, Bangiomorpha, from the 1.2 Ga Hunting
Formation of Canada. (a) A colony of whiskery filaments growing from holdfasts attached to a

limestone base. (b) A single filament showing a single-series filament making a transition to

multiple series, with sets of four wedge-shaped cells; note the sets of four disk-shaped cells in the
single-series part of the strand. (Courtesy of Nick Butterfi eld.)

Whereas some of the Bitter Springs Chert Molecular phylogenies (see Fig. 8.4) show
fossils were once supposed to show meiotic that many lines of eukaryotes have tradition-
cell division, and so sex, this is now doubted. ally been termed “algae”. Several algal groups

Must paleontologists find fossils of early now seem to be closely related to true plants
eukaryotes actually engaged in sexual repro- (see p. 483). The fossil record of algae is patchy,
duction in order to prove the origin of sex? but exceptions are the biostratigraphically

The answer is no, and a phylogenetic argu- useful dinoflagellates, coccoliths and diatoms,
ment is enough. If we know that all species in and calcareous algae such as dasycladaceans,
a modern clade show sexual reproduction, charophytes and corallines (see p. 221).
then their ancestors probably did too. Many Why have sex? Budding seems to be effi -
modern algae show sexual reproduction, and cient enough, and it is what Bacteria, Archaea
the oldest member of a sexually reproducing and many simple eukaryotes have always
group is a 1.2 Ga red alga (see p. 200), so that done, and continue to do today. The benefi ts
provides a minimum date for the origin of are that the process is quick and effi cient:
sex. what could be better for a successful organism
One of the oldest multicellular organisms is than to replicate identical clones of itself? Sex,
Bangiomorpha (Box 8.3), obviously multicel- on the other hand, is a messy and complex
lular and a member of a modern group that business. Many simple organisms, and even
engages in sex. Multicellularity allowed many fishes and amphibians, produce vast numbers

new forms to appear. The term “algae” refers of eggs, sometimes millions that are shed into
to a paraphyletic assortment of single-celled the water, where most are wasted. Sperm of
and multicelled organisms, all of them eukary- course is also produced in vast quantities, and
otes, and most of them photosynthetic. The most goes to waste. Nonetheless, the inven-
major groups are distinguished by their color, tion of sex is usually seen as one of the great
morphology and biochemical properties. milestones in biological evolution (see p. 546).