Benton M.J. Introduction to paleobiology and the fossil record

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178 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

EXTINCTION THEN AND NOW

Extinction events

Somewhere between background extinction

and mass extinction have been many times

when rather large numbers of species have

died out, but perhaps only in one part of the

world, or perhaps affecting only one or two

ecological groups. These medium-sized extinc-

tions are often classed together as extinction

events, but clearly each one is different. Many

extinction events have been identiﬁ ed (see Fig.

7.2), and some of the better-known ones are

noted brieﬂ y here.

The ﬁ rst is the Ediacaran event, about

542 Ma, which is ill deﬁ ned in terms of timing,

but it marks the end of the Ediacaran animals

(see pp. 242–7). Some Ediacaran beasts may

have survived into the Cambrian, but the

majority of those strange quilted jellyﬁ sh-like,

frond-like and worm-like creatures disap-

peared, and the way was cleared for the dra-

matic radiation of shelly animals at the

beginning of the Cambrian. Because of the

antiquity of this proposed mass extinction, it

is hard to be sure that all species became

extinct at the same time, and some would

argue that this was not a mass extinction at

all. Causes are equally debated, with some

evidence for a nutrient crisis or a major tem-

perature change. An older putative mass

extinction, at the start of the Ediacaran, some

650 Ma, might have been triggered by global

cooling, the “snowball Earth” model (see p.

112), but this is equally debated.

An extinction at the end of the Early Cam-

brian marked the disappearance of previously

widespread archaeocyathan reefs (see p. 268).

A series of extinction events occurred

during the Late Cambrian, perhaps as many

as ﬁ ve, in the interval from 513 to 488 Ma.

There were major changes in the marine

faunas in North America and other parts of

the world, with repeated extinctions of trilo-

bites. Following these, animals in the sea

became much more diverse, and groups such

as articulated brachiopods, corals, ﬁ shes, gas-

tropods and cephalopods diversiﬁ ed dramati-

cally during the great Ordovician radiation

(see p. 253).

There were many further extinction events

or turnover events in the Paleozoic, between

the Late Devonian and PT mass extinctions,

including a substantial extinction phase bet-

ween the Middle and Late Permian, some

10 myr before the PT event. This Middle–Late

Permian extinction, the end-Guadalupian

event, may turn out to be a mass extinction

in its own right. Numerous marine and non-

marine groups were hard-hit at that time, and

it has been hard to identify until recently

because its effects were sometimes confused

with the end-Permian event, because of lack

of clarity about dating.

There were further such events at the end

of the Early Triassic and in the Late Triassic.

The Late Triassic extinction event, more com-

monly called the Carnian-Norian event (after

the stratigraphic stages) occurred some 15–

20 myr before the end-Triassic mass extinc-

tion. The Carnian-Norian event was marked

by turnovers among reef faunas, ammonoids

and echinoderms, but it was particularly

important on land. There were large-scale

changeovers in ﬂ oras, and many amphibian

and reptile groups disappeared, to be followed

by the dramatic rise of the dinosaurs and

pterosaurs. At this time, many modern groups

arrived on the scene, such as turtles, crocodil-

ians, lizard ancestors and mammals. The

cause of these events may have been climatic

changes associated with continental drift. At

that time, the supercontinent Pangaea (see p.

48) was beginning to break up, with the

unzipping of the Central Atlantic between

North America and Africa.

Extinctions during the Jurassic and Creta-

ceous periods were minor. The Early Jurassic

and end-Jurassic events involved losses of

bivalves, gastropods, brachiopods and ammo-

nites as a result of major phases of anoxia.

Free-swimming animals were unaffected, and

the events are undetectable on land – they

may be partly artiﬁ cial results of incomplete

data recording. Events have been postulated

also in the Mid Jurassic and in the Early Cre-

taceous, but they are hard to determine. The

Cenomanian-Turonian extinction event some

94 Ma, associated with extinctions of some

planktonic organisms, as well as the bony

ﬁ shes and ichthyosaurs that fed on them, is

probably associated with sea-level change.

Extinctions since the KT event have been

more modest in scope. The Eocene-Oligocene

events 34 Ma were marked by extinctions

among plankton and open-water bony ﬁ shes

in the sea, and by a major turnover among

MASS EXTINCTIONS AND BIODIVERSITY LOSS 179

mammals in Europe and North America.

Later Cenozoic events are less well deﬁ ned.

There was a dramatic extinction among

mammals in North America in the mid-Oli-

gocene, and minor losses of plankton in the

mid-Miocene, but neither event was large.

Planktonic extinctions occurred during the

Pliocene, and these may be linked to disap-

pearances of bivalves and gastropods in tropi-

cal seas.

The latest extinction event, at the end of

the Pleistocene, while dramatic in human

terms, barely qualiﬁ es for inclusion. As the

great ice sheets withdrew from Europe and

North America, large mammals such as mam-

moths, mastodons, woolly rhinos and giant

ground sloths died out. Some of the extinc-

tions were related to major climatic changes,

and others may have been exacerbated by

human hunting activity. The loss of large

mammal species was, however, minor in glo-

bal terms, amounting to a total loss of less

than 1% of species.

Recovery after mass extinctions

After mass extinctions, the recovery time is

proportional to the magnitude of the event.

Biotic diversity took some 10 myr to recover

after major extinction events such as the Late

Devonian, the end-Triassic and the KT. Recov-

ery time after the massive PT event was much

longer: it took some 100 myr for total global

marine familial diversity to recover to pre-

extinction levels. Species-level diversity may

have recovered sooner, perhaps within 20 or

30 myr, by the Late Triassic. But the deeper

diversity of body plans represented by the

total number of families took much longer.

It is becoming clear that all the rules

change after a profound environmental crisis

(Jablonski 2005). Disaster taxa prove the

point (Fig. 7.12). These are species that, for

whatever reason, are able to thrive in condi-

tions that make other species quail. Stromato-

lites, for example, in marine environments

and ferns on land make sudden but brief

appearances. After the PT crisis, the inarticu-

lated brachiopod Lingula ﬂ ourished for a

brief spell, before retiring to the wings. Lingula

is sometimes called a “living fossil” because

it is a genus that has been known for most of

the past 500 myr, and it lives today in low-

oxygen estuarine muds. Other post-extinction

disaster taxa in the earliest Triassic are the

bivalves Claraia, Unionites and Promyalina,

found in black, anoxic shales everywhere.

These animals could presumably cope with

poorly oxygenated waters.

Bivalves and brachiopods diversiﬁ ed slowly

in the next 5–10 myr, as did the ammonoids.

But other groups had gone forever. The rugose

and tabulate corals and other Late Permian

reef-builders had been obliterated. The “reef

gap” following the PT mass extinction is pro-

found evidence for a major environmental

crisis. The rich tropical reefs of the Late

Permian had all gone, and nothing faintly

resembling a coral reef was seen for 10 myr

after the event. When the ﬁ rst tentative reefs

reassembled themselves in the Middle Trias-

sic, they were composed of a motley selection

of Permian survivors, a few species of bryo-

zoans, stony algae and sponges. It took

another 10 myr before corals began to build

true structural reefs (see p. 289).

The reef gap in the sea is paralleled by the

“coal gap” on land. Coals are formed from

dead plants, and there were rich coal depo-

sits formed through the Carboniferous and

Permian, indicating the presence of lush

forests. After the acid rain had cleared the

land of plant life, no coal formed during the

Figure 7.12 Disaster taxa after the end-Permian

mass extinction: the brachiopod Lingula (a), and

the bivalves Claraia (b), Eumorphotis (c),

Unionites (d) and Promyalina (e). These were

some of the few species to survive the end-

Permian crisis, and they dominated the black

anoxic seabed mudstones for many thousands of

years after the event.

(a) (b) (c)

(d) (e)

180 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

ﬁ rst 20–25 myr of the Triassic. It was only in

the Late Triassic that forests reappeared. Tet-

rapods on land had been similarly affected,

and ecosystems remained incomplete and

unbalanced through the Early and Middle

Triassic until they rebuilt themselves in the

Late Triassic with dinosaurs and other new

groups (see p. 454).

Life recovers slowly after mass extinctions.

A ﬂ urry of evolution happens initially among

disaster taxa, species that can cope with harsh

conditions and that can speciate fast. These

disaster taxa are then replaced by other species

that last longer and begin to rebuild the

complex ecosystems that existed before the

mass extinction. The mass extinction crisis

may have affected life in two ways: conditions

after the event may have been so harsh that

nothing could live, and the crisis probably

knocked out all normal ecological and evolu-

tionary processes.

Extinction today

We started this chapter with the dodo, a rep-

resentative of how humans cause extinction.

There is no question that the extinction of the

dodo was regrettable, as is the extinction of

any species. But where should we stand on

this? Some commentators declare that we are

in the middle of an irreversible decline in

species numbers, that humans are killing 70

species a day, and that most of life will be

gone in a few hundred years. Others declare

that extinction is a normal part of evolution,

and that there is nothing out of the ordinary

happening.

The present rate of extinction can be cal-

culated for some groups from historic records.

For birds and mammals, groups that have

always been heavily studied, the exact date of

extinction of many species is known from

historic records. The last dodo was seen on

Mauritius in 1681. By 1693, it was gone, prey

to passing sailors who valued its ﬂ esh, despite

the fact that it was “hard and greasie”. The

last Great auks were collected in the North

Atlantic in 1844 – ironically, the last two

Great auks were beaten to death on Eldey

Island off Iceland by natural history collec-

tors. Some sightings were reported in 1852,

but these were not conﬁ rmed.

Human activity has not simply caused the

extinction of rare or isolated birds. The last

Passenger pigeon, named Martha, died at

Cincinnati Zoo in 1914. Only 100 years

earlier, the great ornithologist John James

Audubon, had reported a ﬂ ock of Passenger

pigeons in Kentucky that took 3 days to go

by. He estimated that the birds passed him at

the rate of 1000 million in 3 h. The sky was

black with them in all directions. They were

wiped out by a program of systematic shoot-

ing, which, at its height, blackened the land-

scape with Passenger pigeon carcasses as far

as the eye could see.

These datable extinctions can be plotted

(Fig. 7.13) to show the rates of extinction of

birds, mammals and some other groups in

historic time. The current rate of extinction

of bird species is 1.75 per year (about 1% of

extant birds lost since 1600). If this rate of

loss is extrapolated to all 20–100 million

living species, then the current rate of extinc-

tion is 5000–25,000 per year, or 13.7–68.5

per day. With 20–100 million species on

Earth, this means that all of life, including

presumably Homo sapiens, will be extinct in

800–20,000 years. These ﬁ gures are startling

and they are often quoted to compare the

present rate of species loss to the mass extinc-

tions of the past.

A reasonable response to this calculation

would be to query the annual loss ﬁ gure and

the validity of extrapolating. The birds that

have been killed so far are mainly vulnerable

species that lived in small populations on

150

100

1600 1700 1800 1900 2000

Year

Number oif recorded extinctions

Figure 7.13 The rate of historic extinctions of

species for which information exists, counted in

50-year bins. Note the rapid rise in numbers of

extinctions in the period 1900–1950; the

apparent drop in the period 1950–2000 is

artiﬁ cial because complete counts have not been

made for that 50-year period yet.

MASS EXTINCTIONS AND BIODIVERSITY LOSS 181

single islands (e.g. the dodo) or in extreme

conditions (e.g. the Great auk). Perhaps more

widespread species such as pigeons, sparrows

and chickens will survive such depredations?

But recall the Passenger pigeon – it should

have been immune to extinction. The other

point is to query whether it is right to extrap-

olate the ﬁ gures from bird and mammal

extinctions to the rest of life. Species of birds

and mammal are short-lived (i.e. they evolve

fast), and perhaps their extinction rates are

not appropriate for insects and plants, for

example.

The jury is still out on modern extinction.

It is clear that surging human population and

increasing tension between development and

ecology put pressure on natural habitats and

on species. Plants and animals are dying out

faster now than at times in the past when the

global human population was smaller. Pale-

ontologists and ecologists have an important

job to do in seeking to understand just what

the threats are and how fast the modern

extinction is proceeding.

Review questions

1 How do paleontologists and other earth

scientists study mass extinctions? Carry

out a census of papers about the Permo-

Triassic event published in the last year.

Find the ﬁ rst 50 papers using any biblio-

graphic search tool, and classify them by

broad theme (paleontology, stratigraphy,

geochemistry, atmospheric modeling, vol-

canology), geographic region (perhaps by

continents), sedimentary regime (marine,

terrestrial) and key conclusion about the

extinction model (eruption of Siberian

Traps, gas hydrate release, acid rain,

anoxia, meteorite impact). How are our

views perhaps biased by limited geo-

graphic coverage, a major focus on marine

rocks and dominant academic discipline?

Are these biases to be expected, and

why?

2 Is there any evidence that the media dis-

torts research agendas? Look at news

stories about the KT event, and consider

the balance of reporting of different

aspects: do a census of the animal and

plant groups mentioned in the ﬁ rst 50

news reports you encounter.

3 Investigate one of the “other” mass extinc-

tions not covered in detail here: end-Ordo-

vician, Late Devonian and end-Triassic.

4 Calculate the relative magnitudes of the

big ﬁ ve events from Jack Sepkoski’s data-

base of fossil genera, either through http://

strata.ummp.lsa.umich.edu/jack/ or http://

geology.isu.edu/FossilPlot/.

5 Why is the current loss of species on

Earth sometimes termed the “sixth

extinction”?

Further reading

Benton, M.J. 2003. When Life Nearly Died. W.W.

Norton, New York.

Benton, M.J. & Twitchett, R.J. 2003. How to kill

(almost) all life: the end-Permian extinction event.

Trends in Ecology and Evolution 18, 358–65.

Briggs, D.E.G. & Crowther, P.R. 2001. Palaeobiology,

A Synthesis, 2nd edn. Blackwell, Oxford, UK.

Erwin, D.H. 2006. Extinction: How Life on Earth

Nearly Ended 250 Million Years Ago. Princeton Uni-

versity Press, Princeton, NJ.

Gotelli, N.J. & Colwell, R.K. 2001. Quantifying biodi-

versity: procedures and pitfalls in the measurement

and comparison of species richness. Ecology Letters

4, 379–91.

Hallam, A. & Wignall, P.B. 1997. Mass Extinctions and

their Aftermath. Oxford University Press, Oxford,

UK.

Hammer, Ø. & Harper, D.A.T. 2005. Paleontological

Data Analysis. Blackwell Publishing, Oxford, UK.

Jablonski, D. 2005. Mass extinctions and macroevolu-

tion. Paleobiology 31, 192–210.

Taylor, P. 2004. Extinctions in the History of Life. Cam-

bridge University Press, Cambridge, UK, 204 pp.

References

Alvarez, L.W., Alvarez, W., Asaro, F. & Michel, H.V.

1980 Extraterrestrial cause for the Cretaceous-

Tertiary extinction. Science 208, 1095–108.

Bambach, R.K. 2006. Phanerozoic biodiversity mass

extinctions. Annual Review of Earth and Planetary

Sciences 34, 127–55.

Benton, M.J. 1995. Diversiﬁ cation and extinction in the

history of life. Science 268, 52–8.

Hammer, Ø. & Harper, D.A.T. 2005. Paleontological

Data Analysis. Blackwell Publishing, Oxford, UK.

Jablonski, D. 2005. Mass extinctions and macroevolu-

tion. Paleobiology 31, 192–210.

Jin, Y.G., Wang, Y., Wang, W., Shang, Q.H., Cao, C.Q.

& Erwin D.H. 2000. Pattern of marine mass extinc-

tion near the Permian-Triassic boundary in South

China. Science 289, 432–6.

182 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Keller, G., Barrera, E., Schmitz, B. & Mattson, E. 1993.

Gradual mass extinction, species survivorship, and

long-term environmental changes across the Creta-

ceous-Tertiary boundary in high latitudes. Bulletin of

the Geological Society of America 105, 979–97.

McKinney, M.L. 1995. Extinction selectivity among

lower taxa – gradational patterns and rarefaction error

in extinction estimates. Paleobiology 21, 300–13.

Orth, C.J., Gilmore, J.S., Knight, J.D., Pillmore, C.L.,

Tschudy, R.H. & Fassett, J.E. 1981. An Ir abundance

anomaly at the palynological Cretaceous-Tertiary

boundary in northern New Mexico. Science 214,

1341–3.

Raup, D.M. 1979. Size of the Permo-Triassic bottleneck

and its evolutionary implications. Science 206,

217–18.

Raup, D.M. & Sepkoski Jr., J.J. 1984. Periodicities

of extinctions in the geologic past. Proceedings

of the National Academy of Sciences, USA 81,

801–5.

Rohde, R.A. & Muller, R.A. 2005. Cycles in fossil

diversity. Nature 434, 208–10.

Twitchett, R.J. 2006. The Late Permian mass extinction

event and recovery: biological catastrophe in a green-

house world. In Sammonds, P.M. & Thompson, J.

M.T. (eds) From Earthquakes to Global Warming.

Royal Society Series on Advances in Science No. 2.

World Scientiﬁ c Publishing, Hackensack, NJ, pp.

69–90.

Wignall, P.B. & Twitchett, R.J. 1996 Oceanic anoxia

and the end Permian mass extinction. Science 272,

1155–8.

Chapter 8

The origin of life

Key points

• Life originated by fusion of organic molecules in the ﬁ rst billion years after the forma-

tion of the Earth.

• The precursor to living cells may have been self-replicating RNA; a time before life

originated termed “RNA world”.

• Photosynthesis by a group of bacteria, called cyanobacteria, generated molecular oxygen

), and the atmosphere became oxygenated at a low level 2.4 Ga. Later, oxygen levels

increased further, around 0.8–0.6 Ga.

• The universal tree of life, reconstructed from gene sequencing of modern organisms,

shows there are three great domains: Bacteria, Archaea and Eucarya. The ﬁ rst two are

prokaryotes, the last eukaryotes.

• The earliest fossils are bacteria in rocks up to 3.2 Ga, indicated by stromatolites, struc-

tures built by alternating algal mats and sediment layers.

• Cellular fossils 3.5 Ga are highly controversial; the ﬁ rst widely accepted cellular fossils

date from 2.5 Ga.

• Biomarkers, notably lipids, provide evidence for cyanobacteria and eukaryotes

2.7 Ga.

• The oldest eukaryotes, cells with a nucleus and organelles, date back perhaps 1.9 Ga.

• Red algae from 1.2 Ga show that sex had originated – they show mitosis, but also

meiosis, which is unique to sexual reproduction.

• Together with sex came multicellularity, the possession of many, often specialized, cells,

ﬁ rst seen in 1.2 Ga red algae.

Life is improbable, and it may be unique to this planet, but nevertheless it did begin

and it is thus our task to discover how the miracle happened.

Euan Nisbet (1987) The Young Earth

184 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Origins are among the deepest questions:

Where did humans come from? Where did life

itself come from? The most ancient philoso-

phers could see that the world is made of

living and non-living things, and they wanted

to know where the spark of life came from.

How do you go from a non-living thing, like

a rock or a glass of water, to a living thing,

like a plant or an animal?

These early speculations led to many cre-

ation myths, stories about how the non-living

to living transition might have taken place.

Creation myths are common to many reli-

gions, and they explain the origin of life by

divine intervention. These ideas are not scien-

tiﬁ c, however, because they cannot be tested.

We explored the issue of creationism in

Chapter 5.

The current scientiﬁ c view is that life arose

on the Earth some time before 3.5 Ga (Ga =

giga years old, or 1000 Ma). In rocks from

Australia and South Africa dated at around

3.5 Ga, isotopes of carbon are consistent with

the presence of a marine biosphere that pref-

erentially incorporated the carbon-12 (C

)

isotope into organic matter relative to C

The ﬁ rst organisms were simple, single-celled

prokaryotes similar to modern microbes.

More complex cells, eukaryotes, arose only

later, perhaps 2.7 Ga, and much later than

that came the ﬁ rst true plants and animals.

This means that the ﬁ rst three-quarters of

the history of life passed by in the company

of organisms that were neither plant nor

animal.

In this chapter, we look ﬁ rst at different

ways of explaining origins. Then, we go on to

look at the diversity of evidence about when

and how life arose. We concentrate on the

geological and fossil evidence, of course, but

include some necessary molecular biology and

biochemistry as well.

THE ORIGIN OF LIFE

Scientiﬁ c models

There have been many scientiﬁ c models for

the origin of life, some of them now rejected

by the evidence, and others still available as

potentially valid hypotheses:

1 Spontaneous generation.

2 Inorganic model.

3 Extraterrestrial origins.

4 Biochemical model.

5 Hydrothermal model.

Medieval scholars believed that many

organisms sprang into life directly from non-

living matter, a form of spontaneous genera-

tion. For example, frogs were said to arise

from the spring dew and maggots were said

to come to life in rotting ﬂ esh. However,

careful tests proved that there was no truth in

these ideas. Louis Pasteur in 1861 enclosed

pieces of meat in airtight containers, and

maggots did not appear. He showed that ﬂ ies

laid their eggs on rotting meat, the eggs

hatched as maggots and the maggots then

turned into ﬂ ies. So, the idea of the origin of

life by spontaneous generation is a scientiﬁ c

hypothesis because it may be tested, but it

turns out to have been wrong. It is import-

ant to realize that scientiﬁ c and non-scientiﬁ c

do not mean “right” and “wrong”: science is

about testing and rejecting alternate hypoth-

eses until one remains that is not rejected.

The inorganic model for the origin of life

is that complex organic molecules arose grad-

ually on a pre-existing, non-organic replica-

tion platform – silicate crystals in solution.

Silicate crystals, clay minerals, were subject to

selection pressures on the ancient seabed, and

then organic molecules became involved and

the inorganic selection became organic. This

view has been championed vigorously by

Graham Cairns-Smith of Glasgow University,

but it has not gained widespread support. The

ﬁ rst experiments to test the model were carried

out in 2007, but they were not conclusive.

The extraterrestrial model is that the build-

ing blocks for life were seeded on Earth from

outer space. Simple molecules, such as hydro-

gen cyanide, formic acid, aldehydes and acet-

ylenes are found in certain classes of meteorites

called carbonaceous chondrites, as well as in

comets, and these chemicals might have been

delivered to the surface of the Earth during a

phase of massive meteorite bombardment

about 3.8 Ga. In other, more extreme, forms

of this hypothesis, DNA might even exist in

space, or life in its entirety might have evolved

elsewhere in the universe, and was seeded on

the Earth during the Precambrian.

Collectively, these views have sometimes

been called “panspermia”, meaning “univer-

sal seeding”. The panspermia model received

THE ORIGIN OF LIFE 185

a boost in 1996 when David McKay and a

team from NASA announced that they had

identiﬁ ed fossil bacteria and organic chemical

traces of former life in a Martian meteorite.

These ﬁ ndings have, however, been disputed

vigorously, and the initial excitement has

waned. It is hard to see how extraterrestrial/

panspermia models for the origin of life could

be tested decisively and, in any case, posit-

ing the origin of life on another planet still

leaves open the question of how that life

originated.

The biochemical model for the origin of life

was developed in the 1920s independently by

a Russian biochemist, A. I. Oparin, and a

British evolutionary biologist, J. B. S. Haldane.

They argued that life could have arisen

through a series of organic chemical reactions

that produced ever more complex biochemi-

cal structures (Fig. 8.1). They proposed that

common gases in the early Earth atmosphere

combined to form simple organic chemicals,

and that these in turn combined to form more

complex molecules. Then, the complex mole-

cules became separated from the surrounding

medium, and acquired some of the characters

of living organisms. They became able to

absorb nutrients, to grow, to divide (repro-

duce) and so on.

The hydrothermal model is a recently pro-

posed modiﬁ cation to the Oparin–Haldane

biochemical model (Nisbet & Sleep 2001).

According to this view, the last universal

common ancestor of life (sometimes abbrevi-

ated as LUCA) was a hyperthermophile, a

simple organism that lived in unusually hot

conditions. The transition from isolated amino

acids to DNA (Fig. 8.1) may then have hap-

pened in a hot-water system associated with

active volcanoes. There are two main kinds of

hot-water systems on Earth today, hot pools

and fumaroles fed by rainwater that are found

around active volcanoes, and black smokers

in the deep ocean. Black smokers arise along

mid-ocean ridges, where new crust is being

formed from magma welling up as major

oceanic plates move apart (see p. 42). Seawa-

ter leaks down into the crust carrying sulfur

as sulfate, mixes with molten magma and

emerges as superheated steam, with the

sulfur now concentrated as sulﬁ de. As miner-

als precipitate in the cooler sea bottom waters,

they color the emerging hot-water plume

black. Black smokers are too hot as a site

for the origin of life, but the other kinds of

hydrothermal systems are less extreme.

This leaves us the Oparin–Haldane bio-

chemical model as a broad-brush picture of

how life might have originated, and the hydro-

thermal model as a speciﬁ c aspect. How far

have scientists been able to test the biochemi-

cal model?

Testing the biochemical model

In cartoons and pop ﬁ ction, the white-coated

scientist is seen in a laboratory full of mysteri-

ous bubbling glass vessels, and he declares,

“I’ve just created life”. Could this be true?

How far have the experiments gone along the

chain of organic synthesis that is postulated

in the biochemical model for the origin of life

(see Fig. 8.1)?

It took some years before the ﬁ rst labora-

tory results were obtained. The Oparin–

Haldane biochemical model was proposed in

primitive atmosphere

O, N

, H

, CO, H

small- and medium-sized molecules

sugars, purines, pyrimidines, amino acids, lipids

large molecules

polysaccharides, nucleic acids, proteins

membrane

“protocells”

genetic mechanism;

ATP system

prokaryotes

organelles

eukaryotes

distinct tissue

layers

multicellular

organisms

c. 3500 Ma

c. 1600–900 Ma

c. 1260–950 Ma

Figure 8.1 The biochemical theory for the

origin of life, as proposed by I. A. Oparin and

J. B. S. Haldane in the 1920s. Biochemists have

achieved steps 1–3 in the laboratory, but

scientists have so far failed to create life. ATP,

adenosine triphosphate.

186 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

the 1920s, but nobody tested it seriously until

the 1950s. In 1953, Stanley Miller, then a

student at the University of Chicago, made a

model of the Precambrian atmosphere and

ocean in a laboratory glass vessel. He exposed

a mixture of water, nitrogen, carbon monox-

ide and nitrogen to electric sparks, to mimic

lightning, and found a brownish sludge in the

bottle after a few days. This contained sugars,

amino acids and nucleotides. So, Miller had

apparently recreated step 2 in the sequence

(see Fig. 8.1). However, nowadays most

researchers consider the mixture of gases that

Miller used (with high percentage concentra-

tions of H

and CH

) to have been too strongly

chemically reducing to represent a likely

atmosphere for the early Earth. Atmospheric

hydrogen is ultimately replenished from the

mixture of gases released from the solid Earth,

but the geochemistry of the subsurface means

that the mixture generally should contain the

oxidized form of hydrogen (i.e. water vapor,

O) rather than the large proportion of H

in Miller’s atmosphere.

Further experiments in the 1950s and 1960s

led to the production of polypeptides, poly-

saccharides and other larger organic mole-

cules (step 3). Sidney Fox at Florida State

University even succeeded in creating cell-like

structures, in which a soup of organic mole-

cules became enclosed in a membrane (step

4). His “protocells” seemed to feed and divide,

but they did not survive for long.

Could scientists ever show how non-living

protocells could become living? Did this

happen in one jump or was there an interme-

diate stage?

RNA world

Biochemists and molecular biologists have

worried about the transition from non-living

to living; it is hard to see how bacterial cells

could form from non-living chemicals in one

step. What then could have been the transi-

tional form of “precellular” life? The most

widely accepted view today is that RNA is the

precellular entity, and the time between non-

life and life has been termed the “RNA

world”.

RNA, or ribonucleic acid, is one of the

nucleic acids and it has key roles in protein

synthesis. Proteins are manufactured within

the nucleus of eukaryotic cells, and within the

cell mass of prokaryotic cells. The genetic

code, the basic instructions that contain all

the information to construct a living organ-

ism, is encoded in the DNA (deoxyribonucleic

acid) strands that make up the chromosomes.

There are several different forms of RNA that

have different functions: one type acts as the

template for the translation of genes into pro-

teins, another transfers amino acids to the

ribosome (the cell organelle where protein

synthesis takes place) to form proteins, and

a third type translates the transcript into

proteins.

In 1968, Francis Crick (1916–2004), who

co-discovered the double-helix structure of

DNA in 1953 with James Watson, suggested

that RNA was the ﬁ rst genetic molecule. He

argued that RNA must have the unique prop-

erty of acting both as a gene and an enzyme,

so RNA on its own could act as a precursor

of life. When Harvard molecular biologist

Walter Gilbert ﬁ rst used the term “RNA

world” in 1986, the concept was contro-

versial. But the ﬁ rst evidence came soon

after when Sidney Altman and Thomas Cech

independently discovered a kind of RNA

that could edit out unnecessary parts of the

message it carried before delivering it to the

ribosome. Because RNA was acting like an

enzyme, Cech called his discovery a ribozyme.

This was such a major discovery that the two

were awarded the Nobel Prize for Chemistry

in 1989; Altman and Cech had conﬁ rmed part

of Crick’s prediction.

Since 1990, numerous labs have been

chasing evidence for the RNA world. For

example, Jack Szostak and colleagues at Mas-

sachusetts General Hospital in Boston argued

that the ﬁ rst RNA molecules on the prebiotic

(“before life”) Earth were assembled ran-

domly from nucleotides dissolved in rock

pools (Szostak et al. 2001). Among the mil-

lions of short RNA molecules, there would

have been one or two that could copy them-

selves, an ability that soon made them the

dominant RNA on the planet. To take this

forward to create a living cell, Szostak identi-

ﬁ ed two stages: (i) the production of a proto-

cell by the combination of an RNA replicase

and a self-replicating vesicle; and (ii) the pro-

duction of a cell by the addition of a living

function (Fig. 8.2).

Simply proving that RNA could act as gene

and enzyme was one thing; however, a single

THE ORIGIN OF LIFE 187

molecule cannot both replicate and trigger

that replication. The minimum requirement is

that two RNA molecules interact, one to act

as the enzyme to bring together the compo-

nents, and the other to act as the gene/tem-

plate. Together the template and the enzyme

RNA combine as an RNA replicase. But these

components have to be kept together inside

some form of compartment or cell, otherwise

they would only occasionally come into

contact to work together. Szostak and

colleagues then proposed there must be a

second precellular structure they call a self-

replicating vesicle, a membrane-bound struc-

ture composed mainly of lipids (organic

compounds that are not soluble in water,

including fats) that self-replicates, or grows

and divides from time to time. The RNA rep-

licase at some point entered a self-replicating

vesicle, and this allowed the RNA replicase to

function efﬁ ciently.

This is a protocell, but it is not yet living.

It is just a self-replicating membrane bag with

an independent self-replicating molecule

inside. To make the protocell function as an

integrated cell, the RNA replicase has to carry

out a function that beneﬁ ts the membrane

component. For example, the RNA repli-

case might generate lipids for the membrane

through the medium of a ribozyme. With the

membrane keeping the RNA replicase together

and so improving its function, and the RNA

replicase producing lipids for the membrane,

the protocell has become a cell. The two func-

tions are coupled, and the cell can evolve, as

vesicles with improved ribozymes can grow

and divide, and become more abundant than

others. So, we have life and we have evolu-

tion. The cell is alive because it has the ability

to feed itself, to grow and to replicate. Evolu-

tion can happen because the cells show dif-

ferential survival (“survival of the ﬁ ttest”),

and the genetic information for replication is

coded in the RNA.

A number of researchers have carried out

experiments to explore all these steps in the

hypothetical RNA world model. They have

succeeded in evolving ribozymes capable of a

broad class of catalytic reactions, including

linking components of RNA and lipid mole-

cules, and over time the molecules are selected

to perform more efﬁ ciently. Much work has

yet to be done to show how the whole process

could have worked, especially to improve the

efﬁ ciency and accuracy of copying from the

template. The other aspect of the model is

the self-replicating vesicle. Experiments here

have focused on simple physical models for

how oily droplets might incorporate free-

ﬂ oating lipids, and so grow, and then how the

droplets or vesicles might divide when they

reach a certain size or when external forces

are applied, perhaps by the movement of

self-replicating

vesicle

replicase

protocell

linking function

(e.g. ribozyme)

cell

Figure 8.2 The model behind “RNA world”,

where an RNA replicase and a self-replicating

membrane-bound vesicle combine to form a

protocell. Inside the vesicle, the RNA replicase

functions, and might add a function to improve

the production of the vesicle wall through a

ribozyme. At this point, the RNA replicase and

the vesicle are functioning together, and the

protocell has become a living cell, capable of

nutrition, growth, reproduction and evolution.

Read a general introduction to RNA world at

http://www.blackwellpublishing.com/

paleobiology/. (Based on information in Szostak

et al. 2001.)