LateCambrian

490m

PRISCOAN ARCHEAN PROTEROZOIC

CRYPTOZOIC AEONS

MESOZOIC

CAINOZOIC

Late Carboniferous300m

Late Cretaceous 65m

Last glacial maximum18,000

■ Last occurrence or

extinction of trilobites

■ First

appea

rance

of

echinoid

s, bryo

zoan

s

■ First appearance

of jawed fish

■ First appearance of

land plants

■First appearance of

primates

■ Diversif

icatio

n of

mammals and bird

s

■ Fi

rst ap

pear

ance

of Eq

uida

e

■ F

irst a

ppea

ranc

e of

gras

ses

■

Maj

or

ex

tinct

ion

of

mar

ine

and

non-

m

arin

e or

gani

sms

■ Last

occu

rrence

of

rugose

corals

and or

thid

brachiop

ods

■ Major thermal event, temperature up to 5,500°C

3750 First appearance of stromatolites

Middle Silurian420m

E O C E N E OLIGOCENE

PALA

EOG

ENE

JURASSIC

CRETACEOUS

MIOCENE PLIOCENE PLEISTOCENE

PERM

I A N TRIASSIC

MiddleOrdovician

465m

Late Permian250

CAMBRIAN

ORDOVICIAN

CARBONIFEROUS DEVONIAN S I L UR

I AN

■ Last appearance of banded ironstone■ First appearance of common red-beds■ First multicellular organisms ■ Ediacara fauna

■ First appearance of hard skeletal material■ First appearance of many invertebrate phyla■ First appearance of vertebrates (jawless fishes), grapolites

■ Last appearance of graptolites

■ First appearance of pelycosaurs■ First appearance of winged insects■ Proto-Atlantic ocean finally closes

■ Rajmahal Traps (115m)■ Tasman Sea opens■ First appearance of marsupiuals, monotremes

■ First appearance of birds ■ India, Madagascar,Antarctica separate■ Last occurrence of labyrinthodonts■ First appearance of angiosperms (flowering plants)

■ First appearance of hexacorals, dinosaurs and marine vertabrates■ Diversification of labyrinthodont amphibians■ South Atlantic opens

■ Beginning of Labrador Sea opening■ Rapid diversification of mammal-like reptiles, thecodonts■ First appearance of diatoms ■ Australia and Antarctica begin separating slowly

■ First appearance of Globigerinacea foraminifera■ Gondwana and Laurasia separate■ Central Atlantic opens

■ First appearance of hominids■ Expansion of grasslands and grassland vertebrates■ Desiccation of Mediterranean Sea

■ Australopitheus africanus■ Major northern hemisphere glaciations■ Neanderthal man■ Megafauna extinction

■ First appearance of amphibians (labyrinthodonts)

■ Oldest datedrocks on earth■ Oldest sedimentaryrocks■ First micro-organisms,first photosynthesis

PALAEOZOICPA

LAEO

ZOIC

MES

OZOIC

Oligocene 25m

Present

■ Deccan Traps

■ Last occurrance of

rudists, amm

onites and

non-avian dinosaurs

LateJurrassic

150m

1.78million

years ago

4600million

years ago

545million

years ago

250million

years ago

65million

years ago

Early Devonian390m

THEDAWNOFLIFE

S GEOLOGISTS go further andfurther back in time, they have foundthat our planet used to be a verydifferent, unfamiliar and even hostileplace. Some 4100 million to 4200

million years ago, when the oldest known rocks onEarth were forming, the planet would have beenbleak indeed. It was a hellish time, just as the namefor this period suggests — Hadean —after Hades, the Underworld. Volcanoeswere blasting out lava fountains, meteorsand meteorites were crashing into theEarth’s surface, there was really no stablesurface on which soil could form, no liquidwater, and the air was filled with unpleasantgases. Nothing lived here.

As time passed, though, gases and waterproduced by the volcanic eruptions and, to alesser extent, by incoming comets, formedoceans and a more pleasant atmospherebegan to take shape. In this cauldron, lifegot its start.

Thanks to the discovery of small zircon crystalsfrom Mount Narrayer in Western Australia, whichwere radio-dated, we know that 3800 million yearsago the Earth’s crust was forming. Many differentkinds of sediments were being deposited and wehave found ripple marks showing that water waspresent then. And in that water lived some of theearliest forms of life we know of.

Oxygen and ironDuring these early times virtually all of our irondeposits were laid down, very likely with theassistance of primitive bacteria. These are the so-called Banded Iron Formations that have given usthe great iron mines of Western Australia andelsewhere.

We know that when these iron depositsappeared, the Earth hadvery little oxygen in the

atmosphere. But by 1800million years ago,such big iron deposits

were no longer beinglaid down in any greatquantity, apparentlybecause oxygen wasbuilding up in the

atmosphere as living forms (photosynthesisingbacteria and algae) pumped it out in quantity.

Although we depend on these ancient irondeposits for so many modern products, they are nolonger forming. In fact, they have not beenforming in any large quantitiesfor more than 1800 million years.Happily, we are not likely to runout in the near future.

First signs of lifeFrom around 4000 million untilabout 2500 million years ago isthe next big geologic aeon, calledthe Archean. Some of the oldesttraces of fossils left by livingorganisms are stromatolites,

ANDTHERISEOFCOMPLEXITY

■ DR MIKHAIL A.FEDONKIN, HEAD,LABORATORY OFPRECAMBRIANORGANISMS,PALEONTOLOGICALINSTITUTE, RUSSIANACADEMY OFSCIENCES, MOSCOW,AND HONORARYRESEARCH ASSOCIATE,SCHOOL OFGEOSCIENCE,MONASH UNIVERSITY.

■ DR JIM GEHLING,PALAEONTOLOGIST,DIVISION OFNATURAL SCIENCES,SOUTH AUSTRALIANMUSEUM, ADELAIDE.

■ DR PATRICIAVICKERS-RICH, CHAIR INPALAEONTOLOGY,SCHOOL OFGEOSCIENCES,MONASH UNIVERSITY.

structures generally made of calcium carbonate(the same chemical composition as limestone).They are made of material trapped in mats ofgreen algae, cyanobacteria and other kinds ofbacteria. For the most part what you find in the

fossil record are columns andmounds of layered carbonates —but, once in a while, theindividual microscopic fossilisedcells of the life that caused thesestructures to form are actuallyfound trapped within the layers.

Australians can see structureslike these still forming in thebriny bays of Western Australianorth of Perth, especially at aplace called Shark Bay. When youwade into the lagoons of this arid

impressions of animals with many cellsoccur in sandstones more than 600 million

years old. They are called the Ediacara biotaafter the place where they were first discovered.

These primitive marine animals come in agreat variety of shapes and patterns and have beeninterpreted in many different ways. Someresearchers have labelled them relatives of jellyfish,segmented worms, sea pens, sea stars andmolluscs. Some appear to be related to the joint-legged animals, perhaps relatives of themagnificent trilobites that became so abundanttens of millions of years later.

Other fossils from the Flinders are so weirdthat just to whom they are related is very unclear.But we can be sure that there were some movinganimals because they left the first clear tracks, justlike those left by sea snails on the beach after thetide goes out.

The Cambrian explosionAbout 545 million years ago (give or take a fewmillion years), the Earth seemed to explode withbiodiversity. Manygroups of organismsthat we have littlerecord of before andhard skeletonsappear for the firsttime in the fossilrecord. Remains ofonce-living things arepreserved ratherthan beingobliterated. Nolonger are we dependent on just the tracks andtrails or soft imprints left behind by life forms thathad no hard parts.

There are many theories about whyskeletons appeared at that time, but the

reason for this is possibly related to achange in ocean chemistry. Some suggestthat the more ready availability ofphosphate churned up from the bottomwaters of the ocean basins at about thistime kicked off the “skeletonrevolution”.

Another idea is that some organismsbegan eating others and those at the

higher end of the food chain beganconcentrating the makings of hard tissues,

so skeletons began. Perhaps to begin with,skeletons made of calcium carbonate with

a phosphate component served only asstorehouses for phosphatesuseful in the

metabolic activities(for example, energy-producing chemicalreactions) oforganisms.But once skeletons

formed, they providedmany otheradvantages —protection from predators,attachments formuscles and more efficient motion, even

protection from drying out in forms that livedin the zone between tides, and so on.

From the first invertebrate (animalswithout a backbone) skeletons came the

vertebrates (those with backbones, like sharks,dogs and us) and in short order a great variety oflife forms that we know so well today.

coast it is like travelling back billions of years.Apparently the reason these colonies ofmicroorganisms are still doing what they didbillions of years ago is that they are living in anenvironment that excludes other organisms, likesnails, which would eat them. They are some ofthe few living things that can tolerate the salty, aridenvironment of Australia’s west coast.

Life becomes more complexThe life forms that lived during the early history ofthe Earth were quite simple. They were single-celled and had no distinct nucleus to hold all of thegenetic information. They were called procaryotes.Some of these sorts of organisms are still aroundtoday, such as pond scum.

As environments on Earth changed, though,

life became more complex. Cellswith distinct nuclei and cells thathad specialised structures insidethem, like mitochondria,appeared. Some biologistssuggest that this came aboutwhen two cells came togetherwith mutual dependence on eachother’s waste products. This wasa process called symbiosis, withcells stacking one within anotherto form a single working unit.

Then even more complex organisms appearedwith more than one cell — the metazoa, ormulticelled forms.

Australia is again fortunate to have a record ofsome of the world’s oldest complex life forms. Inthe Flinders Range of South Australia, the

Words by:

A

Boom time: the Earth was once covered in volcanoes.

Studying how life beganon Earth can help usbetter understand today’splants and animals. So let’s take a trip backin time…

1PAGES 8-9 EDUCATION

WEDNESDAY, NOVEMBER 13, 2002

SCIENCE SERIES: BIOLOGY

These stromatolites from Western Australia are around

3500 million years old.

An ediacara fossil, one of the firstmulticelled organisms, from theFlinders Range in South Australia.

GRAPHIC: MICHAEL WHITEHEADAND MATT DAVIDSON

ART DIRECTOR:ANDREW WOLF

SERIES EDITOR:ANGUS HOLLAND

THANKS TO: THEMONASH SCIENCECENTRE

Shark Bay sanctuary, home to today’s living stromatolites.

Iron: they stopped making it 1800 million years ago.

An early sea creature called anammonite, discovered in Australia.

At seven million years, this is possiblythe world’s oldest human skull,

found in the Chad desert (thoughsome researchers say it’s an ape).