Anthro notes

2 ANTHRONOTLS

MUSEUM OF NATURAL HISTORY PUBLICATION FOR EDUCATORS

VOLUME

yi

NO.

1

SPRING/SUMMER 1999

HUMAN ORIGINS:

ONE MAN'S SEARCH FOR THE CAUSES IN TIME

by Ruth Osterweis Selig

"Alan

Walker once

said to

me,

It doesnot matter

how much you

can convinceyourself;

itonlymatters

how much you

can convinceyourskeptics'

—

^{thatisscience ina}nutshell."

RickPotts,interview,4/2/99.

Of ask

^allthe

from

^{animalspecies}

whence on

^earth,

they

^only

humans came.

Paleoanthropologistsstrivetoanswerthewhat, the

why and

the

how

ofthatremarkablejourney. In a recent article,

"Why Are We Human?,"

Rick Potts, directorofthe Smithsonian's

Human

OriginsProgram,

summarized

thestateofcurrentknowledge:

Due

to therapidpace ofdiscovery, scientists

now

have fossils

from more

than 5,000 individualsas farbackas5 millionyears.

That

recordoffersstrongevidencethat

we

evolved

from

apelike species in Africa,

and

genetic evidence confirms that our closest biological cousinsarethe Africanchimpanzees.Scientists

from many

differentfields agree that

humans and

chimpanzees evolved

from

a

common

ancestor that lived

between

5 million

and

8 millionyearsago(1999a:l).

Today we know

thatas

many

as 12 to15 different human-likespeciesevolvedinthepast.

Why

did

some

continue

and

change while

most

died out? It is this questionthathas

consumed

RickPotts'life,beginning

when

he

was

aninthgraderinsuburbanPhiladelphia's

Abington High

School.

The

storyofPotts' determin- ation to answer this question reveals

much

about

human

evolution

and

paleoanthropology, but it also offersinsightintoonescientist'ssingle-mindedpassion

and

the development of a

new

theory

—

"variability selection"

—

^toexplainthe

why and

^the

how

^of

human

origins.This ^article presents three intertwinedstories:

•

the development of

one

paleoanthropologist's career;

•

thedevelopment ofthe

human

speciesthrough time;

and

•

the development ofa

new

theory of

human

evolution:variability selection.

Thesethreestoriesilluminate theinextricablenature of scientific advances,

human

knowledge,

and

the individualscientist.Inaddition,the storyof RickPotts underscores theinterplaybetweeninherentdisposition

and

environmental influence,

no

small irony for a scientist

whose

theoryof

human

evolution focuses

on

theinterplaybetweentheenvironment

and

the

human

lineage's evolving predisposition toward adaptability, diversity,

and

versatility.

The

necessity to understand timebothaspersonaltimeduring

which

anindividual

life unfolds

and

^as geologic time within

which

the

human

lineage evolved is another

theme

running throughthethreestories.

INSIDE: URBAN COMMUNITY PROJECT AINU EXHIBIT TEACHER RESOURCES

The

Early Years

Inarecentinterview,Potts tracedhis earliestinterestin origins toplaygrounddiscussionswithhisolderbrother, today a mathematician. Potts'awarenessofa passionfor

human

origins

became

evidentduringa9thgrade

world

civilizationclass:

"When

^Ileft9thgrade,I

knew

I

would become

ananthropologist

and

^thatI

would spend my

lifestudying

human

^{origins in} EastAfrica. I

went

to sleep atnight

dreaming

of doingjust that."

That

9th grade year Pottsaskedhisparents,neitherof

whom had gone

to college, to

buy him two

books:

Desmond

Morris' The

Naked Ape and

Robert Ardrey's African Genesis.

He

^still

remembers

devouringthesections

on

animalbehavior.

"By

the

end

of 9thgradeI

was

completely hooked;

then,inthe1 1th grade, Itookahalf yearanthropology course." His brother

was

studying anthropology at college,

and

theysharedbooks

and

ideas.

A

history

and

a biology teacher supported Potts' ambitions.

"My

history teacher shared

my

passion for understanding time,

and we

talked

many

timesabout the differences between individual

and

geological time."

She

invited Pottstotakethe

AP exam

even

though

hisparentscould not afford thecost (shepaidforthe

exam) and

he

was

not eveninthe

AP

^{historyclass.}

That

experience

and

the

encouragement

of his biology teacher

made

a deep impression

on

Potts.

By

the

end

of12thgrade, in 1

971

Potts chose a local university,

Temple,

to study anthropology

and

pursue his goal of studying

human

origins inEastAfrica.

The Environment

Karl Butzer

had

published his

ground

breaking Environment

and

Archaeology:

An

EcologicalApproachto Prehistory that

same

year. Potts' first anthropology professor at

Temple

believed thestudy ofthe

human

past couldnot beseparated

from

thestudy ofancient ecology

and

assignedallhisstudentstoread Butzer. In hisfirst

two

years,Potts

immersed

himselfinbiological studies,focusing

on

naturalselection

and

adaptation.

He

took courses in cognitive sciences

and

physiology, fascinated

by

theconnection betweenbrainphysiology

and

behavior,realizingtherewas

no way

to separatethe study ofphysical

from

cultural evolution. His senior thesis

on

stonetoolsarguedthat toolscarryinformation not onlyabout

human

capacityfortechnology butalso aboutgeneral

human

behavior.

Comparing

stonetools

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ofsuccessive

hominid

species

meant comparing

cultural

and

behavioral differences

among

species. Allthisseems familiar today, butin the early 1970sstudies ofstone toolsusually

meant

statisticalstudiesdescribing various types oftechnologies,with

no

referencetosuchlarger issuesofbehavior orculture.

Graduate School

Potts

went

straight into graduate school, choosing Harvardprimarilybecauseseveralofitsprofessors called

him

for interviews ("I felt as ifI

was

a sport's

team

recruit; I

had

notheard

much

about

Harvard

given

my

humble

roots,but ^I

was

impressed

by

theirinterestin me.") Neanderthal specialist Erik Trinkaus, then a

young

assistant professor, read Potts' application,

commenting

that"he

seemed

allover theplace."

A

year laterTrinkaus

remembered

his

comment and

toldhim:

"Now

Iunderstand

you want

^tofocus

on

only

one

thing, but

you want

to takeeverything intoaccountto

do

it."

PaleoanthropologistAlan Walker, thenalsoteaching at Harvard,

became

a mentor.

Most

importandy,

he

arrangedforPotts to

do

his

Ph.D.

research

on

Olduvai Gorge, gaining

Mary

Leakey's blessing for the work.

Thus

beganthelastphasein Potts'journeyto

become

a paleoanthropologist

—

^{workinginthefield.Firsthe}

was

to travel to France to gain experience at several

AnthroNotes Volume22

No

1Spring/Summer1999

archaeological excavations, thentoEast Africato

work

both atOlduvai

Gorge

inTanzania

and

at the

Kenya

National

Museum

ⁱⁿ Nairobi, Kenya,to

work on

the materials previously excavated

by Mary and

Louis LeakeyatOlduvai Gorge. RickPotts

was 23

^{years old,}

itwas 1976,

and

he

was

abouttoliveouthis

boyhood

dream.

Potts explained that in the 1960s

and

even the

1970s, thestudy

ofhuman

origins

was

stillaboutfinding

fossilbones

and

analyzing stone tools, particularly for theearly Plio-Pleistoceneperiod of1.5 to2.5 million years ago. Researchers were notyet reallyconcerned aboutbehaviororlandscapes."It

was

awonderful time for

me

tobestartingout,with

my growing

interestsin ecology,behavior

and

naturalselection."

The

Evolution

of

Early

Humans

The

contextforunderstandingPotts'researchactivities over the next

two

decades is the story of

human

evolution, astoryhe has recountedin severalpopular accounts of the process thattransformed a5-million- year-oldtropicalapeinto a

human

speciesofworldwide influence (1999a). Distinctively

human

qualities

emerged

over a periodof about 5 million years rather thanallatonce.

As

Potts explains the dramatic story, walking regularly

on two

legs(bipedalism)

was

thefirstbigstep, forever alteringthe

way

ourancestors interactedwith

their environment.

By

4 million years ago, apelike individuals (the australopithecines)

had

evolved

who

werebipedalbutretainedanabilitytoclimbtrees.Their brains were about one-third the size of a

modern

human's,theyweighed between

60

to108 pounds,

and

theirheightrangedfrom3.5 to5feettall.

Either

among

theaustralopithecinesortheearliest

members

ofour

own

genus

Homo,

stone toolmaking begantobe

common by

about2.5

miUion

years ago.

The

earliest fossils of

Homo

areat least2.3 to2.5 million years old, a time period that also sees an increase in cranialorbraincapacity.

By

1.9millionyears ago,the species

H.

erectus

had

reached

modern human

size

and body

proportions and

was

fully

committed

tobipedal walking.H.erectuswasthefirst

hominid

to leave Africa, spreading toAsia

by

about 1.6 million years ago. H.

erectus'brain size, however,

was

not fully

human, on

averageonlyabouttwo-thirds tothree-fourths thatof

fossil

modern humans. The

relationshipofH.erectus to thevariousspeciesbeforeit isstillhotly debated.

How

later fossil

humans

with

modern

brainsizearerelatedto ourselvesisalsocontroversial,particularly inthecaseof the Neanderthals. (For

more

information

on

these currentcontroversies, seeBrooks,1998a,b.)

Only

afterthe brain

had

reached

modern

size

do we

see the

complex

behaviors

we

associate with being

human:

art, clothing,

complex

stone technology, symbolicrepresentation,

and

religiousbehaviorssuch^as burial.

These emerged

only within the past 100,000 years.

Although

thereis

no

complete agreement

among

scientists to explain the

emergence

offully

modern humans,

it isagreedthatourspecies,

modern H.

sapiens, has

been

the only

human

species

on

Earthforat leastthe last

25,000

^{years. Itis}only withinthepast10,000years that farming

and

herding, cities, writing, trade

and

warfarearose.

As

should be clear

from

this brief synopsis, humanity's features

emerged

over time; there

was no

singlethreshold or step

when humans

originated.

Olduvai Gorge

It

was

to

examine

early stone tool

development

that Potts traveledtoEast Africa to

do

the researchforhis

Ph .D

.In1

977

hearrivedinNairobi,

Kenya

to re-analyze thefossilbones

and

stonetoolsdiscovered

and

described

by

theLeakeys,aswellastoanalyzeother data

from Bed

I ofOlduvai Gorge. It

was Mary and

LouisLeakey's

work

at Olduvai

Gorge

(1936-1985) that

had

helped shape scientific

and

popular ideas about the earliest origins of

human

behavior. It

was

they

who

tried to identifythe

maker

oftheearly

"Oldowan

stonetools"

and

toclarify early

hominid

technology

and

activity at

Olduvai Gorge, then considered the world's oldest archaeologicalsite.

Today,

the

Oldowan

industry has been dated to at least 2.5 million years at older East African sites

and

lasts with little change for about 1

millionyears.

To some

specialistssuchas

Glynn

Isaac, thepivotalquestioninthearchaeologyofearly

humans was

to explain

"how

high density clusters of stone artifacts

and animals bones were found

together"(l 994:8).

New taphonomic

studies

had begun

to

document

the processes

by which

fossilbones

and

associatedstone artifacts

were

deposited,

damaged, and

buriedovertime.

Processes such as water transport or feeding

by

carnivorescouldalter

what

archaeologistsfoundmillions of years later and, therefore, could influence archaeological inferencesabout

hominid

activity. Potts

and

his colleague Pat

Shipman conducted

a groundbreaking study

on

bones

from

Olduvai

Gorge

using the scanning electron microscope,

comparing

marks

on

^fossil bones with

marks produced by known

causes (such as carnivore activity or

damage from

excavation)

on modern

bones. In the journal Nature Potts published his first major scientific paper, describing

how

stonetool

marks

could bedistinguished from

damage

to bones

made by

carnivores

and

other taphonomicprocesses.

With

aclearerunderstandingof

human and

carnivoretoothmarks,Potts

now had

a

way

of seeing

how

early

human

toolmakers

and

carnivores

had

overlappedorinteracted (1981).

He

concentrated

on

thehominids'abilityto

make and

transport toolsover longdistances,freeing

them from

theapes'"eat as

you

go"survivalstrategy.

The

hominids'transportingtools

and

food to a single place

was

acritical transition to creating single places of rest, later

known

as

"home

basecampsites."

Potts' first book, Early

Hominid

Activities at

Olduvai

(1988), summarizeshisdetailed re-analysisof Olduvai

hominid

behavior that explainedsiteformation.

Four

levelsof analysisaredetailed:

How

did thesite

form?

What

did

humans do

there?

How

did thedifferent sites atOlduvai

reflect different activity patterns in space?

How

did the hominids'

activitieschange throughtime?

In thelate1970s, whilePottswas preparing his Ph.D. dissertation,

Glynn

Isaacpublishedhisinfluential articles describing

home

base sites, places

where hominids

apparently gathered together over

one

million years ago to share food

and

tools (1978).

Other

anthropologists

had

been studying

home

base behavior

amongmodern

hunters andgatherers,

and

Isaac proposed an analogy between thesesocieties

and

the early

hominid

ancestors.Inhisdissertation, Potts used studies of

taphonomy,

water transport,

and

landscape analysis to challenge Isaac'sview. ContrarytoIsaac,Pottsconcludedthatthe Olduvaisitesdidnotrepresent

home

bases;instead, the earliest

hominid

^sitesatOlduvai

came

before

home

base development.Based

on

hisre-evaluationofthe

Oldowan

material, Potts assertedthat

hominids

collectedstone materials

and

partsofanimalcarcasses,obtained through scavenging

and

hunting,

and

left

them

at specified locations,so-called"stone caches,"forfutureprocessing.

Infact,carnivoreslikeleopards

and

hyenas,attracted to the carcassremains,

would

have preventedthe use of thesesites

by hominids

asthe placesofprimarysocial activityimplied

by

the

home

basetheory. Pottscalled hishypothesis "resourcetransporter984, 1991).It

was

amajortheoretical breakthrough,

made

beforehe

had

his

Ph.D.

in hand,

and

it

was

well received

by

older colleaguesinthefield,includingIsaac.

In

1983

Potts, then

an

assistant professor of anthropologyatYale University,returnedto Africa,this

rimetodirectpaleontological/archaeologicalexcavations

AnthroNotesVolume 22

No

1Spring/Summer1999

at

Lainymok,

Kenya.

Mary

Leakey traveled

down

to visitPotts'excavation,and,impressedwithhiswork,she suggestedheturnhisattentiontothe

much

^largerareaof

Olorgesailie (Oh-lorg-eh-SIGH-lee).

The

following year Potts gained permission

from

Richard Leakey, DirectoroftheNational

Museums

inKenya,to

work

long-term at Olorgesailie.

By 1985

^theSmithsonian's National

Museum

ofNaturalHistory

had

hiredPotts to starta

new Human

^Origins

Program

attheInstitution.

Within

ayear,

he

wrotethefirstof

many

grantproposals to

fund

large-scale excavations at Olorgesailie. Potts' career

was

launched.

Olorgesailie

For anaspiringpaleoanthropologist,Olduvai

Gorge

was a

dream come

true,but^it

was

Olorgesailiethatchanged Potts'life. In thebeginninghe

was

afterbones

and

tools

and

theopportunitytotest

some

ideasregarding

home

bases

by

enlargingthecontextof

hominid

behavior.

To do

thiswork,Potts

began

todevelopa landscape-scale approachtothe excavation

and

studyof

hominid

tools, animalsbones

and

theoverallenvironment.

At

asingle level,

990,000

years old, Potts'

team

excavated

many

sitesincluding a

huge

elephantbutcherysite. Forseveral

summers

Potts'

team worked

toreconstructthelifeways

and

environmental context of

H.

erectus, 1 millionto

600,000

years ago. Potts differentiated his approach

from

fossil collecting; he

was

searching to understand the ecological niche ofearly

humans by

^focusing

on

excavatinganentirelandscape,notjustsurfacecollecting acrossthelandorputtinganotherfossilontothe family tree.

Soon, however, Potts

had begun

to ask

new

questions regarding space

and

rime.

What

was the ecologyofthe regionthroughtime?

What

habitatsdid the varioushominidslivingtherehavetocope withover a millionyears intime?

With

the

new

datesavailableat Olorgesailie, Potts realizedhecould

document

anentire sequence

from

1.2 million

up

to

49,000

years ago. It

was,hesaid,"an archive ofenvironments,atextbook of

hominid

behavior...theRift Valley writlargethrough time."

To

analyze

and

assess thisremarkable"archive throughtime," Potts againassembled aninternational teamtoexcavate

and

synthesize the

complex

data. Potts has always stressed a

team

approach,

working

with

"geologists, archaeologists, paleontologists,

environmental scientists,

and

a great

group

ofwell- trained

Kenyan

excavators."

Environmental

Oscillation

By

the early1990s environmentalissues

had come

tothe forefront ofpublic attention

and

scientific concern, resulting in

new

research

and

multiple techniques to measureenvironments. Pottsadaptedthese techniques tounderstandingpastclimates,environments,soils,

and

vegetation

—

^applying

many

^ofthese

new

techniques to his amazing "slice through time." (1998a: 96-104).

What

he

found was

starding.

The

dating

and

stratigraphic analysis at Olorgesailie uncovered a widening variability through time. Furthermore, Olorgesailie

was

the tantalizing lead-in to

an examination of

the larger global picture

of

environmentalchange.

Looking

atthe incontrovertible evidence

from

soils, vegetation,

and

lake sediments worldwide, Pottscould

no

longeravoidthe key word:

oscillation.

By 1992

Potts

had become committed

to understandingtheimpactof environmentalchange

on

earlyhominids.

Much

ofthatyearPottsspent walking

up and down

thehillsidesofthesite."Icould

walk up

a hillside

and

see the bands of the blinding white sedimentsofthe lakereplaced

by

grey

and brown

soils followed

by

^{thethinsalt layerindicatingthe lake}

had

driedup.

But

thenalittlefurtherforwardintime, the lake

would

be back.

You walk up and down and

the oscillationofthe

environment becomes

unmistakable,

and you

realize that that

was

the challenge to the hominids, theoscillationoftheenvironment."

But how

didthatchallenge operate?

East African

Mammal Study

In1

992

Pottsthought constandy abouttheextreme environmentalvariability

he saw

asthecruciblethrough

which

the

human

lineage

had

passed.

He

keptasking himself

where had

^allour

human

versatility

and

diversity

come

from;

and

^if

and why humans had

evolved differendyfromother animals,

whose

evolutionhe

had

studiedforyears."Ifnaturalselectionisgoingto

hone

an organism'scharacteristicsto thespecificenvironmentin

which

it lives, then

how do you

transform a small population ofapelike

hominids

into a speciesof world- wide influence, diverse

and

extremelyflexiblein their

behavior.

That

is the critical ecological question of

human

evolution."

Understanding the adaptive challenges for other East African

mammals might

beakey.

As

Potts explains, with

humans we

have

no

comparison,

we

have the unique situation of

human

evolution,

and

our only comparisoniswithearlier

hominids who

didnotsurvive.

So

Pottsturnedto astudy oflarge

mammals

inAfrica, re-analyzing thefossilanimals

from

Lainymok,alarge

and

diverse sample ideal for such a study. Together Olorgesailie

and Lainymok

spanthetimeperiodduring

which modern human

brainsizedeveloped

—

^acritical

timeperiodfor

human

evolution.Potts

and

a colleague Alan

Deino

published their analysis

documenting

the extinction of an entire

group

of

mammals

duringthis period,

around 400,000

years ago (1995).

They

hypothesizedthat large

numbers

of

mammals became

extinctas aresponse torapidclimaticfluctuations

and

extremedietary specialization.

As

the

mammals

eating coarse, low-lying vegetation

became

extinct, smaller,

more

versatile,

and more

generalized eaters emerged;

these arethelarge

mammal

speciesstillwith us today.

Potts

wondered

ifthe

human

lineage

had

gone through a similar pattern of extinction and adaptation as a responsetoextreme environmentalchange.

A New Book

In

1990

Potts

had

signed a contract with William

Morrow

publishers to write a

book

dealing with Olorgesailie

and

environmentalchange.

He

wrotehalf the

book and

thenin

1992

^realizedhisentirethinking

was

shifting. "I

had

a series ofbrainstorms at night,

wondering

if

and how

environmental oscillation

had

been the major influence

on

the developing

human

lineage. I realizedI

had

^tostartthe

book

over again. I

called

my

^{editors, told}

them

^I

was

throwing out everythingI

had

written,butIpromisedtostartover. I

knew

Iwas onto

something

big,

and

thatit

would

take time to

work

out the details. All the training I

had

in college

and

graduate school,allthe earlyconversationsI

had

with

my

brother,^all

my

readingofCharles

Darwin

flooded back. I

knew

^I

had

todealwiththe question of environmental variability

and

its impact as a major selectivefactorexplaining

human

evolution."

Potts realized immediately he

would

have to challenge

one

ofthemajortheories

and

assumptionsof

human

evolution: the transition in Africa from

widespreadforeststowidespreadsavannagrasslandsas the major explanatory factor for the

emergence

of bipedal, tool using

human

beings. In his

new

book,

Humanity

'sDescent(1996a)^,

he

proposedinstead a

new

theoryof environmentalvariabilityasthekeyselective factorexplaining the

emergence

ofthe

human

^species.

The Savannah Hypothesis

According to conventional

wisdom,

our earliest ancestorswereforcedtoadapttoanew,driersavannah environment that replaced a once heavily forested landscape. Bipedal walking developed as a favored adaptation to the ground, with

hominids

usingtheir

newly

freedhandsto

make and

usetools,especially for hunting. Thisled to the eatingof

meat and

increased sourcesofprotein thatfueledalarger brain.Eventually foodsharing,

home

baseliving,social interaction,

and

division of labor

by

sex emerged.

The

savannah hypothesis,

which

Potts

was

originallytaught,

had made

a lot of sense, but it didn't fit the environmental fluctuations thathe

had documented. Over

the short run

some hominids may

have adapted to specific environments including the savannah,

but

over the millennia of time, the

human

lineage

had

to

accommodate

to

and

cope with

huge

oscillations or swingsin the

environment

thatwere manifested^allover theworld(1998a:109-112).

Variability Selection

It

was

this variabilitythatPottsidentified asthekeyto the threedistinctivebreakthroughs

ofhuman

evolution:

bipedal walking ^(4-1.9 million years ago); stone tool

making

(2.5 million-1.5 million years ago);

and

increased brainsize(between

700,000

to1

50,000

years ago)

—

each coinciding with increased environmental

oscillation.

There was

alarger

amount

of savannahin certain areasofthe world,butincreased fluctuationwas just as

much

ahallmark ofglobal climate

and much more

influential

on

the course of

human

evolution.

Potts called his

new

theory "variability selection," a process that links adaptivechange to large degrees of environmental variability.

The

theory refers to variabilityasaselectionagency,nottothevariabilityor versatilitythatdevelopedⁱⁿthe

human

population.

As

Pottsexplains, variability selection"is,in essence, a hypothesisabout

how hominid

evolution hasbeena responsetoenvironments

and

environmentalchange."

AnthroNotesVolume22

No

1Spring/Summer1999

Afteryearsofreconstructingenvironmentalvariability, as well as the evolution

and

extinction of

mammal

species,Pottsrealizedthat survivalofaversatilespecies capableofadjusting tonovelsurroundings

was

the story of

human

evolution.

As

environmental conditions drastically fluctuated, the evolutionary winners

were

populations thatevolvedacapacitytorespondin

new ways

to diverse habitats. This process

—

variability selection

—

^{favors genes that}

improve

^{an organism's}

adaptability,

and

thetheoryexplains

why

ourparticular patternof

human

evolution occurred.

As

Pottssays, "it isnotthe

whole

explanation,butIbelieveitisacritical piece."

Variability selection is also a theory that Potts

knows

modifies

one

of the tenets of

Darwinian

evolution:long-termdirectionalconsistencyinselection over time, consistency implied, for example, in the savannahhypothesis. Ina1

996

articlein Science,Potts explainsthesignificanceofhistheorytounderstanding

human

evolution: "Hardly just noise, long-term fluctuationwasasignalofpotentiallymajorevolutionary consequence. I have proposed the term variability selection to describe the effects ofrepeated, dramatic shifting in Darwinian selection over time. This inconsistencyover

many

generations

may

have

had

an important impact

on hominid

evolution" (1996b:922).

NaturalSelection

As

Pottsexplains ina recentarticle:"Natural Selectionis

theprocess

by which

adaptivestructures areevolved

and

maintained.

As

a result of this century's

union

of populationbiology, genetics,andpaleontology(theneo- Darwiniansynthesis),naturalselectionisregardedasthe

main

causeofchangeinorganisms in relation to their surroundings"(1998b:81). Traditionally, this

meant

consistency of adaptation over time. "Selective consistency, orlong-termuniformselection pressure,is largely

assumed

to be the

way by which

adaptive complexity evolves" (1998b:81).

But

the adaptive conditionsof

hominid

evolution over time, accordingto Potts' research, were highly inconsistent

on

alocal to globalscale.

Potts' theory posits that inconsistency of environmentalconditions

had

criticalimplicationsfor

hominid

evolution.

There

are several

ways

organisms canrespondtohabitatfluctuation(1998b:84-85).

The

firstissimplytofollow the preferred environment, an

Oo>

^S .^DAPTAdILITj "W

^{\sey ?}

adaptive patternthat

works

for awhile,but canleadto extinction

when

largeenvironmentalfluctuations occur.

The

secondistobroadentherangeofconditionsunder

which

an

organism can

live. This can be achieved

by

both genetic

polymorphism

(several different genetic potentials existingwithin the

same

population) or

by

phenotypic plasticity,

when

organisms can respond differendyat

any

given timewith the

same

genotypic inheritance.

A

thirdavenue

of

flexibilityis"variabilityselection,"

or the evolution of adaptive

mechanisms

within a population

which

"assist an organism's sophisticated intake

of and

responsiveness to environmental data"

(1998b:85).

Examples

of such adaptive

mechanisms might

be

new locomotor

systems (suchasbipedalism)

and

anenlarged braintoprocess

and

generate

complex

cognitive responses. In light of Potts' theory, the evolutionofthebraintakes

on new

meaning,asitisour brainthatenables us toadapt tochangingconditions, novel problems,

and

multiple solutions. Climatic oscillation

becomes more

intenseafter

700,000

yearsago

and

it is

from

thistimetoabout150,000yearsagothat the

human

brain reaches

modern

size. Pottshasstated that his theory requires that

we may

have to

"significantly revise the

way

natural selection is

construed to have operated

—

not merely as selection pressureorasadaptationtoa

model

environment, butas

aresponse tohabitat

and

resourcevariability

from

place toplace

and

overtime"(1994:23).

Potts points out that in each

epoch

of

human

evolution, therewerespecies thatevolvedas specialists, that adapted to a specific

environment and

lived alongside

more

versatile forms that survived after the

more

specializedforms

became

extinct.

Two

examples hecitesarethe robust australopithecines

and

the cold- weather adaptedNeanderthals,bothhighlyspecialized hominidsthat

became

extinct.

The

eventualsurvivalof a singleversatile lineage, extraordinarilydiverse in its

behavior

and

spreadworldwide,

may

wellhave evolved

as the result of adaptation to our planet's variable environment.

Nature and Humankind

The

relationshipof nature

and humankind

is

one

ofthe recurring

and most

thought provokingthemes ofPotts' popular

and

extremelywell-writtenvolume,

Humanity

's

Descent(1996a)."Itisimportanttogettherelationship betweenNature

and humankind

right,bothinitslong- term

development and

in its present possibility" (44).

Potts explains that

humans'

penchant for setting themselves apart

from Nature

stems

from

an illusory divideintonatural

and human

domains...adividethat has "never existed over the long course of

human

presence

on

Earth"(267).

The

implicationsofthiskey insighthaveprofoundpublic policyramifications,for, as Pottssays,"theworld

now

rests...

on

the legacyleft

by

a single species" (44)^.

The

fact thatouressential

human

qualities

emerged

as the result of our ecological relationship to

Nature

contrasts with the ironic observation that our resulting

dominion

today could disruptforevertheecologicalbalance

on

Earth.

Conclusion

Science

moves

forward

by

the process of hypothesis testing, development of

new

theories, unearthing

new

data,

and

proposingalternative explanations. Inscience, thedevelopment ofamajor

new

hypothesis or theoryis

always extraordinary in its originality, but it is,

nonetheless,alsoabeginning; colleagueswilltestsuch

new

ideas with their

own

data

and

^their

own

understanding.

For

Potts, Olorgesailie

was

the inspiration, environmental

change

the key to the

development

ofamajor

new

theorytoexplain

human

evolution.Variability selectionisadramaticinsight

and

a theory thatothers

now must

take intoaccountin their attempt to explain the

human

past. Potts

and

hisfellow paleoanthropologists will develop other insights,

and

modificationsofPotts'theoryofvariabilityselectionwill inevitablydevelopthroughtime.

But

theimportance of this

new and

provocativetheorywillstandasamajor contributionto theongoing study of

human

origins.

What

Potts' journey demonstrates is that

one

scientist'sapproachtounderstandingthe

world around him

can

grow from many

seeds: a sharp, fertile

mind

with a

penchant

ofits

own

for "the bigpicture"; an enduring life-longpassion tofind out

where humans came

from;

and

adeterminationtounravel the

whole

puzzle,notjust a single piece.

The

influenceofteachers, mentors,colleagues,

and

thescientists

who came

before,

allinfluencedPottsashe developed through_{time. Just}

as his theory connects the

development

ofthe

human

lineage tothe millenniaofchallengingenvironments,so one canseePotts'lifedeveloping

from

theinteractionof hisunique

mind and

drivingpassionwiththeinfluences ofhisfamily,teachers,colleagues,

and

experiences

—

^his

environment

throughtime.

Postscript

As

^thisAnthroNotesarticle

was

beingwritten,Pottsflew to

London

to present his theory of variability selection-attheinvitationoftheLinneanSociety. Potts

must

havebeenawarethat inJuly¹

858

Charles

Darwin and

Alfred Wallace, at the urging of the geologist Charles Lyell

and

the botanist Sir Joseph Hooker, presented simultaneously their papers

on

evolution throughnaturalselection

—

^{to theLinneanSociety.}

On

May

24, 1859,

Thomas

Bell,presidentoftheSociety, reported, in his presidential address, that

"The

year whichhaspassed.,.hasnot.,.been

markedby

any ofthose strikingdiscoveries

which

at oncerevolutionize, so to speak, thedepartment ofscience

on which

they bear"

{I860: viii). Fortheimpact ofvariability selection,as

withthe theory ofnatural selection,onlytimewilltell.

Further

Reading

Authored by

RickPotts(RichardPotts):

1981."Cutmarks

Made

by StoneToolsonBones from Olduvai Gorge, Tanzania," (withPat Shipman). Nature 291:

577-80.

AnthroNotesVolume 22

No

1Spring/Summer1999

1984.

Home

Bases andEarly Hominids. American Scientist

72:338-347.

1988. EarlyHominidActivitiesat Olduvai. Aldine de Gruyter.

1989. Ecological Context and Explanations of Hominid Evolution. Ossa14:99-112.

1991.

Why

theOldowan?Plio-PleistoceneToolmaking andthe TransportofResources.Journal_ofAnthropological Research 47(2):153-176.

1994."Variables versusModels ofEarlyPleistoceneHominid LandUse."Journal of

Human

Evolution27: 7-24.

1995. Mid-PleistoceneChangeinLarge

Mammal

Faunasof East Africa (with Alan Deino). Quaternary Research 43:106-113.

1996a. Humanity's Descent: The Consequences of^Ecological

Instability.William

Morrow

and Co.

1996b.Evolution and Climate Variability. Science. 273:

922-923.

1998a Environmental Hypotheses ofHominin Evolution.

Yearbookof^PhysicalAnthropology 4\: 93-136.

MJU^-'f

1998b "Variability Selection in

Hominid

Evolution."

EvolutionaryAnthropology 7:81-96.

1999a

Why

Are

We Human?

The WashingtonPost. 4/14/99- HorizonSection: 1,4-5.

1999b

Human

^{Evolution,inEncarta}Encyclopedia. Microsoft Corp.OnlineEncyclopedia.Prehsitory. Aldine-Atherton.

Other

Publications

Ardrey,Robert. 1961. African^{Genesis.Collins.}

Bell, Thomas. 1860. "The Year

Which

Has Passed."

Presidential address to the Linnean Society on the Anniversary oflinnaeus'sbirth,

May

24,1859(Proceedings of^theLinneanSocietyofLondon,^viii-xx.

Bound

inJournal of^theProceedingsofThe Linnean^Society:Botany4).

Brooks,AlisonS. 1998a. "Modern

Human

Origins:What's

New

WithWhat'sOld?"Revisedand updatedarticlefrom AnthroNotes14(3),1992,\nAnthropologyExplored:TheBest ofSmithsonian AnthroNotes, editedbyRuthOsterweisSelig and MarilynR.London. Smithsonian Institution Press,

54-65.

Brooks, Alison S. 1998b. "What's

New

in Early

Human

Evolution5 to1MillionYearsAgo?" Revisedand updated

article from AnthroNotes 18(2), 1996, in Anthropology Explored:TheBestofSmithsonian AnthroNotes, editedby RuthOsterweisSeligand MarilynR.London, Smithsonian InstitutionPress, 38-53.

Butzer,Karl. \91\.EnvironmentandArchaeology:

An

Ecological Approachto Prehistory. Aldine-Atherton.

Isaac, Glynn. 1978. "The Food-Sharing Behavior ofProto-

human

Hominids."ScientificAmerican238: 90-108.

Morris,Desmond. 1967. The

Naked

Ape.McGraw-Hill.

**For further references, request the bibliography

"Human

Evolution and Paleoanthropology ^" from the Anthropology OutreachOffice,SmithsonianInstitution,Washington, D.C.

20560-0112.

Ruth

O. Selig

AnthroNotes

Editor

Selig.Ruth@nmnh.si.edu

wrrV A^e Mug v^lwctvj ?