Lecture notes
Cambrian Explosion
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- University College London (UCL)
Covers the Cambrian Explosion lecture - extra reading with sources cited
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Cambrian ExplosionBackground to the Cambrian Explosion
Ediacaran Epoch – early radiation of metazoan
- Ediacaran fauna (600 – 570 mya) contains first evidence of many modern phyla
- Notable fossils representing modern phyla (eg. Dickinsonia – corals, Charnia –
Pennatulacaea, Kimberella – Mollusc)
- Most fauna almost entirely soft-bodied, no heavily shelled creatures molluscs
and arthropod-like creatures had soft skeletons (unmineralised skeletons)
Pre-cambrian/Cambrian boundary (543 mya)
- Animals (as we known them today) appeared in the fossil record very abruptly at
this point
- Sudden emergence = Cambrian explosion
- Many explanations for Cambrian explosion – increase [oxygen], end of ice age,
genetic bottlenecks
- Ca from ocean ridges allow skeletons to form
- No evidence for Cambrian animals before explosions – why?
a) They were minute (don’t fossilise well)
b) They were cryptic (rare)
c) Arose very rapidly from an ediacaran lineage – time when Cambrian animals
arose is debateable
The Burgess Shale - animals that have lived on or near vicinity of massive submarine
carbonate wall, Cathedral escarpment
- Mid Cambrian – 20mya after
- Aysheia: lobopod – stem group of arthropods, segmented (Opabinia, Santacaris,
marella)
- Pikaia: proto-chordate, Wiwaxia (annelid)
Early attempts to date divergence of metazoan groups (eg. bilaterians –
protostomes and deuterostomes) gave
surprisingly deep divergence times >1 Gya
- What explains the apparent 300 Myr
gap in deuterostome fossil record?
Stem lineages could have originated
and persisted for longperiods of time
before diversification of modern
groups (Hedges, 2001)
- Recent geochemical studies suggested
that major shifts in seawater ion
concentration particularly an increase in calcium occurred ~545 Mya (Brennan,
Lowenstein, Horita, 2004) – increased availiability of calcium may have spurred
, the evolution of biocalcification in different animal lineages that had originated
much earlier (Blair and Hedges, 2005)
Convergence of molecular and fossil evidence – interpretation of some Ediacaran
fossils as not only metazoans but bilaterians consistent with recent phylogenetic
trees that place major diversification of bilaterians in the Ediacaran period
(Peterson et al., 2008)
How can we explain the abruptness of the fossil record?
Simple explanation: rise in atmospheric oxygen – but animals did rise before
atmospheric oxygen rose (Preston cloud)
- Constrained by occurrence and disappearance of BIFs suggested a rise in
atmospheric oxygen (but attempts to plot time-course almost completely
imaginary)
Why oxygen?
Oxygen is paramagmetic – has unpaired electrons (Hund’s rule states that each
electron first fills empty orbitals with similar energy = degenerate), before pairing
in a half-filled orbital
- Oxygen must accept electrons singly, as from ferrous iron (hence rust) –
however, only a limited number of single electron donors in environment, why
ferruginous oceans must be anoxic
- There is a kinetic barrier to reaction with molecules that donate electron pairs
(such as reduced carbon) – the barrier allows oxygen to collect in the
atmosphere to high levels without reacting
- This in turns favours environmental redox equilibria – presence in the
environment at the same time of compounds with both low and high redox
potential - Unique in the respect that it can accumulate in large volumes without
reacting
The significance and uses of oxygen
An electron acceptor: The actual ATP yield per glucose molecule varies between
28-36 ATPs, depending on coupling, other uses of the PMR and ATP/ADP
transporter etc.
- Actual yield of ATP per glucose from some forms of fermentation (not Embden-
Mayerhof pathway) is 3-4
- Aerobic respiration gives a maximum of about 1 order of magnitidue more ATP
than fermentation
- There are other electron acceptors eg. nitrate give ATP yields closer to oxygen,
much higher than fermentation
, Nitrate redox potential is more positive than oxygen (can obtain more ATP through
anaerobic processes – aerobic is not really a process by which you get maximum or
highest yield of ATP) BUT nitrate cannot accumulate in the atmosphere like oxygen
can
- There are some electron acceptors such as nitrate, N 2O, NO have similar redox
potential to oxygen: 600 -1200mV – however, their accumulation of Earth
requires oxygen
- Oxygen is also not needed in protein synthesis, RNA, DNA etc. – as opposed to
the use of nitrate
- Increasing oxygen as e- acceptor, but oxygen accumulation can also lead to
accumulation of other chemicals (sulphate, nitrates – very low concentrations in
anoxic world)
Production of major connective tissues of plants (lignin) and animals (collagen) can
only be produced in presence of free oxygen
- BUT it could be possible to produce such proteins without oxygen – lack of
oxygen in this aspect would not prevent the evolution of animals
- Hydroxylation of proline and lysine key steps in biosynthesis of collagen – both
steps require molecular oxygen (apparent Km for oxygen in proline hydroxylase
system of chick embryo is 3 x 10-5M, equivalent to 2.6% oxygen
Towe (1970) arugues that the rise of large animals was limited by low oxygen
levels before the Cambrian
Connects to preston cloud – need large amounts of oxygen for evolution of more
complex and larger organisms
- But is oxygen is not necessary to form cross-links in collagen – is collagen itself
necessary?
- Lignin in plants also uses oxygen but hard to see it as a necessity rather than
expedient
- Bacteria can produce stearols – do not need a large amount of oxygen to produce
- Oxygen used in many other metabolic pathways but again often anaerobic
equivalents
- Oxygen toxicity leads to clumping of cells, which may encourage multicellularity,
but so do many other factors such as osmotrophy as a feeding strategy
Against the need for oxygen
Organ systems/ subtissue-grade multicellularity has really only evolved twice (in
animals and 200 Myr later in plants) (Butterfield, 2009)
- Environmental threshold or “permissive environment” hypotheses are expected
to yield massively polyphyletic radiations once the limiting conditions have been
met
- Atmospheric pO2 does not explain absence of pre-Ediacaran forests
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