THE HiSToRy oF LiFE 439
chromosomes and a mitotic spindle. Most eukaryotes undergo meiosis, the highly
organized segregation and recombination of genes that is the basis of sexual repro-
duction. Almost all eukaryotes have mitochondria (vestigial and nonfunctional in
a few), and many have chloroplasts. Mitochondria and chloroplasts are descended from
bacteria that were ingested, and later became intracellular symbionts (endosym-
bionts) in protoeukaryotes: another major transition in evolution (see Figure 2.5)
[59, 62]. These events are the most important cases of endosymbiosis, which has
evolved many times in the history of life [67]. Molecular phylogenetic studies show
that the Eukarya are nested within a clade of Archaea that has some of the key
eukaryote genes, such as those that encode actin, tubulin, and other components
of the eukaryotic cell’s cytoskeleton [26, 86, 97]. The most recent common ancestor
of Archaea and Eukaryota probably was capable of phagocytosis, the likely basis of
the capture of the bacteria that became the mitochondrion.
The earliest eukaryote fossils are about 1.8 Gy old, which is consistent with
estimates of the date of the common ancestor of eukaryotes derived from DNA
sequence comparisons [70]. If there were eukaryotes before then (as some chemi-
cal evidence suggests), they left no living descendants. For nearly a billion years
after their origin, almost all eukaryotes seem to have been unicellular, and most
lineages remain so (FIGURE 17.4). Based on cellular characteristics shared among
diverse living eukaryotes, the reconstructed last common ancestor of eukaryotes
was a highly complex cell, combining components that were derived from its
archaeal and bacterial ancestors and components that evolved during the more
than 1 Gy that separated it from the first eukaryotic ancestor [16]. The last common
ancestor of eukaryotes had sophisticated metabolic capabilities, elaborate endo-
membranes, a cytoskeletal system based on actinomysin and tubulin, meiosis, and
a nucleus with nucleocytoplasmic transport [51].
Unicellular and multicellular eukaryotes share most protein-coding gene fami-
lies and regulatory control of gene expression by transcription factors (see Chapter
15). Multicellularity has evolved many times (FIGURE 17.5). Simple multicellularFIGURE 17.4 Some Proterozoic unicellular eukaryotes
from 780–740 My-old strata in the Grand Canyon, Ari-
zona. The phylogenetic relationships of these forms to
living eukaryotes are not well understood, because they
have few distinctive morphological features and do not
have extractable DNA. (A) Trigonocyrillium horodyskyii.
The globular organisms are the acritarchs (B) Valeria
lophostriata, (C) Lanulatisphaera laufeldii, and (D)
Culcitulisphaera revelata. Acritarchs are diverse single-
celled Proterozoic organisms, some of which are related
to diverse algae. (A from [71c]; B–D from [71b].)Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
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Evolution4e_1704.ai Date 11-02-2016(A) (B)(C) (D)17_EVOL4E_CH17.indd 439 3/22/17 1:36 PM