Dna

ribosome cell membrane

aerobicaUy-respirmg bacteria ribosome cell membrane

aerobicaUy-respirmg bacteria a-

prokaryote host cell amoeboid cell cell wall cell membrane mitochondrion

vacuole nucleus containing chloroplast chromosomes vacuole

nucleus containing chloroplast chromosomes

blue-green algae plants with chloroplasts with mitochondria J*2

with mitochondria J*2

ancestral eukaryote with flagellum

spirochaetes animals animals

Figure 8.9 Eukaryote characters: a typical prokaryote cell (a) differs from a eukaryote plant cell (b) in the absence of a nucleus and of organelles. (c) The endosymbiotic theory for the origin of eukaryotes proposes that cell organelles arose by a process of mutually beneficial incorporation of smaller prokaryotes into an amoeba-like prokaryote (steps 1, 2 and 3). (Based on various sources.)

cyanobacteria. So, the amazing thing is that a modern eukaryote cell has proven prokary-otic invaders that possess their own DNA and that coordinate their cell divisions with the divisions of the larger host cell.

Many experts reject the endosymbiotic theory, or at least most of it (Poole & Penny 2007). They point out that the only real evidence for engulfment is for the mitochondria. There is no evidence to support the idea that the nucleus was engulfed, nor is it clear what kind of prokaryote did the engulfing, and in fact engulfment is seen today only among eukaryotes, and not among prokaryotes. So, the alternative view, termed the protoeukary-otic host theory, is that an ancestral eukary-ote, the so-called protoeukaryote, already equipped with a nucleus, indeed did engulf an energy-transferring prokaryote that became the mitochondrion. But this does not tell us where the protoeukaryote itself came from.

Further doubt is cast on the classic endosym-biotic theory by the fact that neither Archaea nor Bacteria appear to be ancestral to Eucarya, and that biomarker evidence indicates an unexpectedly ancient origin for eukaryotes.

Which ever model is correct, when did eukaryotes originate? Molecular evidence about dating the universal tree of life (see Fig. 8.3) has been controversial, but current molecular dates for the evolution of basal eukaryotes appear to be roughly in line with the fossils (Box 8.2).

Basal eukaryotes_

The oldest eukaryote is controversial. Lipid biomarkers indicate that eukaryotes were around at least by 2.7 Ga (see p. 194). The oldest eukaryote fossil may be Grypania, a coiled, spaghetti-like organism that has been reported from rocks as old as 1.85 Ga

(Fig. 8.11a). Slabs are sometimes covered with great loops and coils of Grypania, preserved as thin carbonaceous films. It has been identified as a photosynthetic alga, a type of seaweed, based on its overall shape and, if this identification is correct, it is a eukaryote. Many dispute this identification, and would argue that the oldest eukaryotes are micro-

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