The Major Transitions in
Evolution: A Physiological
Perspective
Andrew H. Knoll
Harvard University
1. Replicating molecules 
Populations of molecules
in compartments
2. Independent replicators 
chromosomes
3. RNA  DNA and proteins
4. Prokaryotes  Eukaryotes
5. Asexual clones  Sexual
reproduction
6. Single cells  Multicellular
organisms
7. Solitary individuals 
Colonies with nonreproductive castes
8. Primate societies 
Human societies
(language)
To
Physiological/Metabolic Major
Transitions
Autotrophy
1. From reliance on abiotic synthesis to chemosynthesis
2. From chemosynthesis to photosynthesis
3. From anoxygenic to oxygenic photosynthesis
4. From reliance on environmental N to nitrogen fixation
Heterotrophy
5. From fermentation to respiration
6. From anaerobic respiration to aerobic respiration
7. From absorption of organic molecules to phagocytosis
8. From diffusion to bulk transport
9. Technology
Photosynthesis
http://en.wikipedia.org/wiki/File:Z-scheme.png
Van Niel Equation: CO2 + 2H2A  CH2O + H2O + 2A
Electron donor can be water, but also Fe2+, As3+, H2S,
H2, organic molecules
Primary production, limited by electron
supply before oxygenic
photosynthesis?
Canfield et al. (2006)
What about early
heterotrophy?
Nealson (1997)
1. Importance of Fe
in Archean
carbon cycle
2. Limitations on
chemoautotrophy
imposed by
oxidant pool
Nealson (1997)
Conceptual model of Archean
and iron formation deposition,
derived from the biological
oceanic iron cycle.
Fischer and Knoll (2009)
Several lines of
evidence indicate
oxygenation 2.4 Ga
• Banded iron formation
• Detrital uraninite, siderite,
and pyrite
• Paleosols
• Sulfur isotopes
Our hero
Plastids 1332/1232
Falcon et al. (2010)
What drove
oxygenation?
2211/2057
Heterocysts
3028/2519
N-fixers
Assumption of
cyanobacterial origins:
3500/2700 Ma
How much O2 accumulated?
Lyons and Reinhard (2009)
Maliva et al. (2005)
Accumulating
oxygen alters
carbon cycle and
its constituent
metabolisms
Nealson (1997)
After Anbar and Knoll (2002)
Shen et al. (2003)
Brocks et al. (2005)
Scott et al. 2008
The Eukaryotic
Cell
1. Qualifies as a major
transition in the
scheme of MS & S.
2. What are its metabolic
or physiological
consequences?
3. Briefly consider
phagocytosis and
the acquisition of
energy metabolisms.
De Duve (2007)
Phagocytosis
1. Enables particle capture,
including bacterial and
protistan cells (and small
animals)
2. Introduces predation as a
key ecological process
3. Changes physical nature of
organic C acquisition, but not
metabolic means of
generating energy
Image shows amoeba eating a yeast cell;
Pierre Casson (http://www.forschung3r.ch)
In eukaryotes, energy metabolism is largely the
product of endosymbiosis, incorporating bacterial cells.
-- Aerobic respiration  mitochondria  proteobacteria
-- Oxygenic photosynthesis  chloroplast  cyanobacteria
Innovation vs. limitation.
Consequences of redox structure for
eukaryotic organisms?
Johnston et al. (2009)
• Mitochondria must have arisen in a global setting where marine
oxygen levels were extremely low and sulfide levels were high.
Furthermore, the first ~1 billion years (at least) of eukaryote
diversification occurred in a marine environment marked by low
oxygen, widespread anoxia and high sulfide.
Martin et al. (2003)
• Hypoxia/anoxia
• Sulfide toxicity (interfere with cytochrome c oxidase in mitochondria)
• Fixed nitrogen availability
Photosynthetic
eukaryotes in midProterozoic oceans
• 0.5 million or more species today
• In mid-Proterozoic oceans,
problematic
• Capacity to fix carbon was not
accompanied by the ability to fix
nitrogen
• In mid-Proterozoic oceans,
limited fixed nitrogen in photic
zone.
• Ecological advantage to
photoautotrophs able to fix N2.
Butterfield (2000)
Mitochondriate eukaryotes in midProterozoic oceans
• Systemic inhibition by sulfide
– interferes with cytochrome
c oxidase function in
mitochondria
• Widespread sulfide in midProterozoic oceans may
have challenged eukaryotes
in many marine
environments.
• Mitochondrial adaptation to
anoxic metabolism occurs
(hydrogenosome, mitosome),
but is a one way street
• When did environmental
challenges of sulfide and
fixed nitrogen fade?
Porter and Knoll (2000)
Subsurface
sulfide decline
• Johnston et al. (2010) – Ferruginous subsurface
waters begin at least 800 Ma, concomitant with
widespread rifting of supercontinent Rodinia
Porter et al. (2003)
Courtesy of
N. Butterfield
ergosterol
Courtesy
of Phoebe
Cohen
24-methyl-5,7,22-trien-3-ol
HO
HO
cycloartenol
HO
-sitosterol
24-ethyl-5-en-3-ol HO
stigmasterol
24-ethyl-5,22-dien-3-ol
More scales…
P. Cohen, PhD thesis
Multicellularity
• A major transition in
MS & S scheme
• But a common
transition – fully 1/3 of
the 119 major
eukaryotic clades
recognized by Adl et
al. (2005) have
evolved simple
multicellularity; most
have limited diversity
• Six (possibly 7)
clades have evolved
complex multicellularity; 95% of all
described eukaryotic
species
The Problem of Diffusion
1. In complex multicellular organisms, only a subset of cells
are in direct contact with the environment.
2. In organisms with 3-D multicelluarity, diffusion will
strongly affect both metabolism and development.
• Diffusion limits size
attainable at any
given pO2
• Circumventing
diffusion:
– Mechanisms to
enhance directional
cell-cell transfer
(plasmodesmata,
gap junctions,
incomplete
septation)
– Specialized cell and
tissue types for bulk
transfer (phloem,
trumpet hyphae,
circulatory systems)
Diffusion and
metabolism
Knoll and Hewitt (2011); left after Runnegar (1991)
• Only surface cells
directly encounter
environment
• Gradient in concentration
of signaling molecules
develops
• Gradient develops in
diffusible environmental
factors that induce cell
differentiation in
unicellular eukaryotes
modification (e.g.,
nutrients, oxygen)
Diffusion and
development
Schlichting (2003)
Development feeds back on physiology
Size
Differentiation
Nutrient/Signal
Gradient
With time, cross a functional threshold that promotes the
diversity (evolvability?) of complex multicellular clades.
MAKES ECOLOGICAL FEEDBACKS POSSIBLE.
Development feeds back on physiology
Size
PO2
Differentiation
Nutrient/Signal
Gradient
With time, cross a functional threshold that promotes the
diversity (evolvability?) of complex multicellular clades.
MAKES ECOLOGICAL FEEDBACKS POSSIBLE.
When did atmosphere/ocean begin its
transition to a more modern state?
Canfield and Teske (1995)
Derry et al. (1992)
Scott et al. (2008)
Dahl et al. (2010)
Ediacaran-Cambrian Animal Radiation
(??)
24-isopropylcholestane; Love et al. (2009)
The
Evolutionary
Present
Peter Brewer (MBARI)
The Punch Line
• Major transitions in
physiology both
track and drive
environmental
changes in Earth
history
• Might characterize
evolutionary
trajectories
wherever life
emerges
Thanks to …
• Members of the Knoll lab (especially Tais
Dahl, Ben Gill and Phoebe Cohen)
• Colleagues further afield, especially Dave
Johnston and Don Canfield
• Funding from NSF, NASA Exobiology, and
the Agouron Institute