g fl» B
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T
W e’re All O ut Of Africa
B y J ane E llen B lair
C ontributing S cience W riter
February is Black H istory M onth
and an excellent tim e to reexam ine
the scientific evidence regarding
the am o u n t o f g en etic d iversity
am ong hum ans and the antiquity o f
that diversity.
ay'& m o d em society,
evolution, mi-
je d in g w hich
hgly m ore di-
i. d em onstrate
’ an d variable we
J id e r the fact
ghiyanability in a
“ 7 o f D rain, Or-
he entire planet.
____Be possib le scien-
Jftons for the tim ing and
l 8 f em erg en ce o f m odem
sa p ien s {H. sa p ie n s) (from
T h ese tire th e O u t o f A r i c a
the Mu •R egional Mdd<S
P a itia lR placem ent I
j has im pliJ
I v a ria b ility !
regarding
hum an opulatio n s
ciple o f v ariab ility is I
Eton dev elo p s in a
11 le longer it exists. Wj
s c i ntific com m unity,
no re a consensus reg ard in g
ditj o f th e d iffe ren t m o d e lsl
lations has v ery deep roots and that
popu latio n s transitioned from a r
chaic to m o d em hum ans in sm all
groups w ithin both A frica and Asia.
In this m odel, the local populations
in Europe, A sia and A frica co n tin
ued their ev o lu tio n from archaic to
anatom ically m o d em H. sapiens in
each area as they developed local
and regional adaptations. C ritics o f
this theory q uestion how so m any
in d e p en d en tly ev o lv in g p o p u la
tions could develop w ith such sim i
lar p hysical characteristics.
P a rtia l R e p la c e m e n t or R e ce n t
A fric a n O rig in s M o d el is essen
tially a hybrid betw een the other
two m ore extrem e views. It is based
on the idea that there w as a sig n ifi
cant am ount o f gene flow or m ig ra
tion betw een populations o f archaic
a lly
m o d e rn
and
anat
leory, there w ere
H.sapiens. In t
lem H. sa p ien s in
anatom ically
1,000 years ago
' A frica about
dispersed due to
w hich gradu
ic and environm en-
changing cli
here w ould have
tai condition
lount o f inbreeding
been a lim it»
w ith resid en t ar-
or bybridi
in E urope and
.OOaic
to replacem ent
lulations. T his
o th a gradual
:ess.
lories state that ana-
Ht ofA frica Theory state
xclu’
i re-
f ar-
SLC’Pe
evolv
a c lt
ns evol
places
Kpe and A sl
lin g the exi]
Jto m ic a lly
uld have in te i
Id y existing gi1
Dmo sapiens. Tf
fe y w ere not acf
ecies. S om e gene
pn) and local selectio
pvented the various 11
i d ev elo p in g into
pd ultim ately resc
variable species.]
km uch higher
nan variabilif
ly
!ped on the
em erged so
and 100,000
siderably 1
en tists’ esti ition). P rio r to that,
>rs, H om o erectus,
our predec
lea and then m igrated
evolved in
g io n approxim ately
outside o f th
its ago. T here m ust
1.8 m illion
least one later tim e
have been
fvolved pop u latio n o f
w hen a mo
the A frican co n ti
H. sa p ien s
n e n c e is not conclu-
nent. The
sive regari [g the degree to w hich
these anat fically m o d e m //, sapi-
w ith p opulations o f
ens inter
apiens, how quickly
archaic
if archaic hum ans were
population
anatom ically m od em
replaced
w hat the pattern s o f
hum ans,
'ere into o th er areas.
m igratio
:al with scientific theo-
As is t
little com m on ground
ries, the
betw een le supporters o f these
Ies.
three th
e several dating tech-
T here
to determ ine w hen an-
niques u
lulations o f H om o sapi-
cestral
ly left the A frican conti-
ens ac
[ecular
tim e clocks are
nent
ites
o
f
m
utation and can
based o
assess
the
age o f hu-
potenti
Isil specim ens, or the rocks
mans,
they
using
cable
500,
longevity o f the
a n d the interbreeding w ith va
archaic populations.
T h e M u lti-R e g io n a l M odel
R egional C on tin u ity M odel implies
that the variability b etw een popu-
lem hum ans devel-
can co n tinent and
ie betw een440,000
ag o (o re v e n c o n
fer ago by som e sci-
o n tain ed in, can be dated
ety o f techniques appli-
le tim escale from 50,000 to
ears ago. M ethods for fos-
[g include uranium series,
lum incsence and electro n
o nance techniques. Each
techniques has different
Ions and ranges o f accuracy
the results to be som ew hat
ious . A dding to the am bigu-
fossil specim ens them selves
sp arse to be con clu siv e
'w hat happened during these
tim e periods.
p u g h the results are incon-
there is som e genetic evi-
ln the Y chrom osom e and on
romosome 21 w hich show s that
there is m ore div ersity am ong A iri
can p o p u latio n s than w ith in non-
A frican p o p u latio n s. T his w ould
be ex pected u n d er th e C o m p lete
R eplacem ent M odel due to the cycle
o f breeding and lo n g ev ity w ithin
A frica, w ith o nly su b sets o f these
peo p le p o p u latin g o th e r areas.
T h e C o m p le te R e p la c e m e n t
M odel also relies h eav ily upon g e
netic sequencing d ata from m ito
chondrial D NA (m tD N A ), w hich is
passed intact form m other to d au g h
ter. T he so c a lled “ M ito c h o n d ria l
E v e” theory sh o w s all m o d em h u
m ans stem m ing from a single A fri
can woman. H ow ever, it is no longer
co n sid ered a v iab le ex p lan atio n by
m ost scientists. In co rrect estim ated
rates o f m u tatio n s, d ifferen t p o p u
lation sizes, m ultiple m igrations and
q u estio n ab le sta tistica l techniques
for identifying p o p u latio n relatio n
ships and d raw ing fam ily trees may
all co ntribute to m tD N A b ein g d is
credited. In ad d itio n , o th e r eq u ally
v alid results and fam ily trees can be
g enerated using th e sam e set o f
data.
T here are sig n ific an t im p lica
tions regarding m o d em hum an vari
ab ility in all o f th ese th eo ries. The
p hrase hum an v ariab ility refers to
how m any po ssib ilities there are for
people to express d ifferen t form s o f
various traits. S in ce scien tists truly
d o n ’t know how q u ick ly the tra n si
tion from archaic to an ato m ically
m od em species occurred, it is d iffi
cu lt to determ in e the am o u n t o f
variability introduced into the popu
lation during th at transition. H o w
ev er, it is clear from m o d em re
search that there is m ore v ariab ility
w ithin a single h um an pop u latio n
th an there is b etw een populations.
S ta tis tic a l/m a th e m a tic a l stu d ies
h a v e b ee n d o n e w h ic h sh o w th e re
is n o se t o f tra its w h ich ca n co n
c lu s iv e ly s e p a ra te m o d e rn h u
m a n s in to d iffe re n t rac es. In fact,
a ll d iv id in g lin es w e c u rre n tly see
a re b a se d o n so c ia l an d c u ltu ral
d iffe re n c e s w ith no rea l b a s is in
s c ie n tific fact.
I t is im p o rta n t to u n d ersta n d
th a t h u m a n v a ria b ility does not
ju s tr e f e r to o u tw a rd a p p earan ces.
O u tw a rd a p p e a r a n c e s are f r e
q u e n tly th e e x p re ssio n o f in te r
n a l tra its. T h e re can be m any
ty p e s o f g e n e tic v a ria b ility b ased
in th e h u m an g en o m e th at are n ot
e x te rn a lly e x p re sse d . W h en c o m
p a rin g h u m an s to c h im p an z ee s,
th e re is a c tu a lly o n ly a 1 '/i%
d iffe re n c e in th e ir g e n e tic code.
A d d itio n a lly , th e D N A in h um an
m ito c h o n d ria is o n ly aro u n d 2
14% as v a ria b le as th a t in ch im
p a n z e e m ito c h o n d ria .
A lth o u g h w e d o n ’t k now the
d e ta ils o f e x a c tly w h en an d how
o u r a n c e s t o r s p o p u l a t e d th e
w o rld , w h at w e d o know is th a t w e
are all o f on e sp e cie s and th e re is
no su c h th in g as d iffe ren t hum an
races. T h ere are m any d an g ero u s
so c ia l co n se q u e n c e s to an y so rt o f
ra c ia l d iv isio n an d th ere is no sci
e n tific ev id en c e to su p p o rt o r en
co u ra g e th is v ie w p o in t. It is c riti
c a lly im p o rta n t th a t a ll p eo p le u n
d e rsta n d the la c k o f sc ien tific ju s
tific a tio n fo r a n y rac ial categ o ries
o r d e sig n a tio n s b ased upon a n y
th in g o th e r th a n cu ltu re and ac
c e ss to re so u rc e s. In to d a y ’s cu l
tu ra lly d iv e rse so c iety , th o se c u l
tu ra l d iffe re n c e s th a t do ex ist b e
tw een g roups sh o u ld be em braced.
CcicÒRArc BtAck hisroRy CDonch
œ
Çet Steady fee ‘Vatentine’i flay.
H
O ur G enetic Link As
Africans
B y P rofessor K kbkca C ann
In the past, anthropologists explored
the history o f our species by using either
bones preserved as fossils or tools and
other artifacts, sue has pots, cloth, metals,
or wooden implements tound m archaeo
logical deposits. In the 1960’s, this focus
began to change, with the recognition by
biologists that we all cany the evidence
o f evolution in our genetic material, and
that modem people can be a key to
unlocking that past The changes in our
genetic material, mutations, that have
accumulated in our DNA over time, can
reveal w here we came from, how we
spread as a species, and when we began
that spread
By 1980, the idea emerged that all
humans alive today can trace at least
some o f their genes to a woman who
probably lived in Africa about 100,000-
200,000 years ago. She wasn’t the only
woman alive at the time, but she repre
sents the only woman who had an unbro
ken line o f female descendants, all the
way to the present Her daughters passed
on a unique set o f genes, mitochondrial
genes, that control the rate at which
energy is produced in all the cells o f our
bodies.
Men and w omen have these genes
but only women transmit them.
Scientists estimate that humans have
about 60,000 different genes, and 37 o f
them fit this pattern. Because they arc
inherited only through the female line,
they are easy to trace and help reveal how
humans dispersed around the wodd
By comparing the patterns o f muta
tions in these genes, and using a com
puter to help reconstruct family trees,
scientists in 1987 published the data for
142 human female lineages from donors
around the world
The tree had 2 major branches, one
that contained only Africans, and the
second that contained Africans and ev-
eryoneelse. The simplest explanation for
tins pattern is that the ancestor of these
people was an African, some people
stayed in Africa and gave rise to modem
African populations, but others migrated
out o f Africa and gave rise to other
continental populations, including a
group that went back into Africa.
Researchers in both fields o f anthro
pology and human genetics w ere aston
ished. Many people assumed tliat earlier
groups o f archaic people would be di
rectly ancestral to the modem popula
tions found in those same places today.
This idea said it wasn’t true, all modem
people shared a recent common origin,
and therefore, the implication was tliat
modem human races were a new phe
nomenon, and the biological basis o f
racial differentiation had to be trivial, ui-
volvingonlyafew genes. These ideas are
still stimulatingmajorresearchprograms
Y
In 1980, Professor Rebecca Cann
o f The University o f Hawai'i at
Manoa found genetic evidence o f
our African origins. Her work was
based on 182 current
mitochondrial DNA types which
pointed to the existence o f a
common female ancestor who
lived 200,00 0years ago in Africa.
on a variety o f levels, including a search
for the dispersal out o f Africa in the
archaeological record, the comparison of
modem human groups for genetic mark
ers that correlate with racial differences,
and the exploration o f cultural and lin
guistic differences that correspond to
racial boundaries, however close or
changing those boundaries may be.
Kofi Agorsah from Portland
State University is screening dirt
with a sieve to look for minute
artifacts at an archaeological
site.
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