Evolution, 4th Edition

THE HiSToRy oF LiFE 437

Recent experiments have shown that clay particles with RNA adsorbed

onto their surfaces can catalyze the formation of a lipid envelope that

can divide into “offspring” envelopes. Protocells might have consisted

of such lipid envelopes containing replicating RNA.

Long RNA sequences would not replicate effectively because the

mutation rate would be too high for them to maintain any identity. A

larger genome might evolve, however, if two or more coupled macro-

molecules each catalyzed the replication of the other. Replication prob-

ably was slow and inexact originally, and only much later acquired the

fidelity that modern organisms display.

How protein enzymes evolved is perhaps the greatest unsolved

problem. This process may have begun when cofactors, consisting of

an amino acid joined to a short oligonucleotide sequence, aided RNA

ribozymes in self-replication [62]. Many current coenzymes have

nucleotide components. RNA ribozymes can also catalyze the forma-

tion of peptide bonds, so the next step may have been the stringing

together of several such amino acid–nucleotide cofactors. Ultimately,

the ribozyme probably evolved into the ribosome, the oligonucle-

otide component of the cofactor into transfer RNA, and the strings

of amino acids into catalytic proteins. Such ensembles of macromol-

ecules, packaged within lipid membranes, may have been precursors of the first

cells—although many other features evolved between that stage and the only cells

we know. The origin of cells is often considered the first of the major evolution-

ary transitions in the history of life, evolutionary changes of major magnitude and

consequence that often lead to an additional level of organization (TAB LE 17. 2).

TABLE 17.2 Six major transitions in the history of evolution leading to higher-level formations, or groups

Major transition Group formed Group transformation

Separate replicators (genes) and

formation of cell membranes →

genome within cell

Compartmentalized genomes Evolution of large, complex genomes

Separate unicells → symbiotic unicell Eukaryotic cells Evolution of symbiotic organelle and nuclear

genomes; transfer of genes between them;

formation of “hybrid genomes”

Asexual unicells → sexual unicells Zygote (sexually reproducing organism) Evolution of meiosis and (often obligate) sexual

reproduction

Unicells → multicellular organism Multicellular organisms Evolution of cell and tissue differentiation and of

somatic vs. germ cells

Multicellular organisms → eusocial

societies

origin of societies (in only a few lin-

eages)

Evolution of reproductive and nonreproductive

castes (e.g., social insects)

Separate species → interspecific

mutualistic associations

origin of interspecific mutualisms Evolution of physically conjoined partners (e.g.,

endosymbioses)

Source: After [11], modified from [62].

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_17.02.ai Date 12-14-2016

5 ʹ

3 ʹ

ACAGAACCUUAAUGC

AAGACAAAU

CCGUG

CA

HO C

C

G

A

A

A

G

pppG – C

G A U U C A A

GGCAG

U U G A G A A

A A C U C U U

• – – – – – –

G A A C A C A

C U U G U G A

• – – – – – A U A U C A C T

U A U A G U G A

• – – – – – – – G G G T C C

C C C A G G

• – – – – –
  
  
  • ––––
  
  
  
  

C

G

G

G

A

G

C

U

C

G U A G G C

G G U A G C

C AA

A A

C A

G G

G U

• – – – – C A U C U U
  C C A G C G – – • –


• 
  


• 
  


• 
  


• 
  

Substrate

FIGURE 17.2 Sequence and structure of the catalytic RNA, a ligase, that evolved

in a simple laboratory system. The oligonucleotide substrate, shown in blue at left,

includes residues that bind to the RNA ligase as shown, as well as a nonbinding loop.

Mutations that occurred during the experiment are indicated in red. The mutations

that were critical for enhanced function are enclosed in boxes. (After [69].)

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