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in prokaryotes. The external membrane of prokaryotes separates digestion of macro-
molecules outside the cell from internal machinery requiring these nutrients. These
primitive cells exert digestion by exporting enzymes, either by secretion or extension
from the cell surface, to hydrolyze complex organic structures. The products of diges-
tion are then processed and ingested through the membrane for metabolic processes
(de Duve, 2007). Compartmentalization of internal cell structures is thought to have
evolved by the regional segregation of digestive enzymes on the external surface of
the plasma membrane, a feature persistent in present-day bacteria. Invagination of
1
Mammals
Reptiles
Amphibians
Actinopterygian fish
Agnathans
Arthropods
Cnidarians
Sponges
Choanoflagellates
Mucorales/Blastocladiales
Hymenomycetes
Archiascomycetes
Candida
Saccharomyes
Plectomycetes
Pyrenomycetes
Amoebozoans
Red algae
Chlorophytan green algae
Mosses
Va scular plants
Alveolates
2
3
4
5
6
7
8
15
14
13
12
16
10
11
9
20
19
18
21
17
Origin of plastids
23
22
Euglenozoans
Diplomonads
2.5
2.0
1.5
1.0
0.5
0
Billion years ago
fIguRe 2.1
A timescale of eukaryote evolution. The times for each node are taken from
the genomic divergence times, except for nodes 1 (310 Ma [million years ago]), 2 (360 Ma),
3 (450 Ma), and 4 (520 Ma), which are from the fossil record; nodes 8 (1450 Ma) and 16
(1587 Ma) are phylogenetically constrained and are the midpoints between adjacent nodes.
Nodes 12 to 14 were similar in time and therefore are shown as a multifurcation at 1000 Ma;
likewise, nodes 21 and 22 are shown as a multifurcation at 1967 Ma. The star indicates the
occurrence of red algae in the fossil record at 1200 Ma, the oldest taxonomically identifiable
eukaryote. Plastid (photosynthesis organelles) origin is shown with an arrow. (From Hedges,
S.B., Blair, J.E., Venturi, M.L., and Shoe, J.L.,
BMC Evol Biol
. 2004 Jan 28;4:2. Open Access
article available at http://www.biomedcentral.com/1471-2148/4/2.)
