Mostly Out of Africa,
but what did the Others have to say?
The migration of modern Homo sapiens (adapted from Cavalli-Sforza &
Feldman, 2003 Nature Genetics 33:266 – 275)
Dan Dediu
Language Evolution and Computation Research Unit
Linguistics and English Language
The University of Edinburgh
April 2006, Rome: Evolang 6, v1.1
D.Dediu@sms.ed.ac.uk
The Question
●
human evolution and language evolution
must be considered together
→ tendency in language evolution to abstract
away;
●
continuous interplay between them
→ discussion focus on (1): influence of human
evolution on language evolution;
●
examples:
→ mode: catastrophic vs gradual [Tim Crow's
(2002) protocadherinXY (Xq21.3/Yp11.2); Lanyon –
this conference]
→ time depth: recent vs ancient [FOXP2;
Johansson – this conference]
Human Evolution:
Deep History (14 – 2 mya)
●
last common ancestor Homo & Pan: ~6 mya
Africa
–
no consensus: Sahelanthropus tchadensis (Brunet et
al., 2002) vs Orrorin tugenensis (Senut et al., 2001) vs
Ardipithecus ramidus kadabba (Haile-Selassie, 2001)
–
common features: chimp-like size, upright
walking, forested habitats
●
radiation of different species of hominins
→ Australopithecus: ~4 mya, small body,
upright walking, small brain, high sexual
dimorphism, tool use (chimp-like?)
→ probably A. anamensis or A. afarensis – our ancestors
→ others: did not left modern descendants (including genera Paranthropus &
Kenyanthropus)
Human Evolution:
Homo erectus
●
genus Homo: ~2 mya, Africa – H. erectus/ergaster [H. habilis is now A. habilis (Jobling, Hurles &
Tyler-Smith, 2004]
–
mainly enlarged brain; 750-1225 cm3 [650 cm3 (Dmanisi D2282, Gabunia et al., 2000) or 600
cm3 (Vekua et al., 2002)]
–
●
overall anatomical similarity to modern humans, large body size, indisputable tool use
history of Homo erectus:
–
born late '40s: Ernst Mayr (subsuming Pitechanthropus, Sinanthropus, Meganthropus &
Telanthropus)
–
1983 (Senckenberg Conference): C. Stringer, P. Andrews & B. Wood: split African (H.
ergaster)
–
vs Asian (H. erectus) → controversy continues today:
balance seems leaning towards a single regionally variable H. erectus species (Jobling,
Hurles & Tyler-Smith, 2004; Kidder & Durband, 2005; Gilbert, White & Asfaw, 2003; Asfaw et al., 2002;
Antón, 2003) → “Daka cranium” (Asfaw et al., 2002), Bouri Formation, Middle Awash, Ethiopia:
transitional form, clusters with Asian specimens.
Human Evolution:
H. erectus - colonizing the Old World
●
before ~1 mya: repeated
expansions (core areas: Rift
Valley, Levant)
intermittent
occupation → climatic
changes (Dennell, 2003)
●
after ~1 mya: stable
populations around OW
●
brain development
similar to moderns
(Mojokerto juvenile; Leigh, in
press)
●
Earliest dates for H. erectus (blue = very early, red = early, green = more recent)
colonization of Flores
island (0.8-0.9 mya) →
language?
Human Evolution:
The Recent Out-of-Africa Model
●
1987: Cann, Stoneking & Wilson, Nature 325:31-36 → mtDNA:
“mitochondrial Eve” (Africa) 140-290 kya
●
1988: Stringer & Andrews, Science 239:1263-1268 → “Noah's
Ark (single origin)”:
–
the ancestral population: recent & (understood to be)
unique → speciation of H.s. in isolated population
–
this population already had the (almost complete)
constellation of anatomical modern features
–
located in Africa
–
initial African differentiation and subsequent
dispersal with replacement
●
supported by most genetic & palaeoanthropological data
(Jobling, Hurles & Tyler-Smith, 2004; Lewin, 1998; etc., etc.)
but:
Problems and Issues:
1. The “Human Revolution”
“By stressing human uniqueness, proponents of the “human revolution”
effectively remove the origin of H. sapiens from the realm of normal
scientific inquiry.” (McBrearty & Brooks, 2000:533)
●
~40-50 kya: sudden appearance of “modern” anatomy & behavior (art, advanced technology,
personal ornaments, long-distance trade)
→ explanations: external memory, contextual focus, integration of
cognitive modules, economic networks, neoteny,
language (Donald, 1999; Klein, 1999; Mithen, 1996; Gabora,
2003; Dunbar, 1996; Bickerton, 2002; Horan, Bulte & Shogren, 2005; Lanyon – this conference)
●
very important critiques: McBrearty & Brooks, 2000 (Journal of Human Evolution 39:453-563),
Henshilwood & Marean, 2003 (Current Anthropology 44:627-651), etc.:
–
defocusing Europe → larger context: demographic effect (intrusive population: Africa via the
Levant)
–
in Africa, the transition was gradual, piecemeal accretion of (morphological &
behavioral - decoupling) modernity over more than 200ky (McBrearty & Brooks, 2000;
Stringer, 2002; Wolpoff & Caspari, 2000; Hawks & Wolpoff, 2001; Trinkaus et al., 2003; Henshilwood &
Marean, 2003; Eswaran, 2002; Johansson – this conference; etc.)
●
no “human revolution”, modernity is not a homogeneous package → no speciation event
Problems and Issues:
2. Origins from a Structured Population
●
Garrigan et al. (Genetics 170:1849-1856),
August 2005:
–
global sample of 42 X (male) chromosomes
→ Xp21.1 locus
–
2 African individuals (Mbuti pygmy):
lineage (non-coding, 17.5 kb) not
recombining with others for > 1my
→ this lineage evolved in isolation
–
the divergence & (recent) admixture of
“(i)f the AMH genome contains any degree
these: in Africa, before expansion into OW
of dual ancestry (i.e., archaic and modern),
then recent African replacement model in
→ rejects species-status claims for H.s. →
its strictest definition (i.e., that of complete
possibility of genetic & cultural contributions
replacement) must be rejected.” (Garrigan et
al., 2005:1855)
from other “archaics”
Problems and Issues:
3. Deep, Non-African
Genetic Lineages
Adapted from Garrigan et al,
2005
HS571B2 (~10kb, non-coding, Xq21.1-33, Yu, Fu & Li, 2002):
non-African specific variant (35% frequency
outside Africa), origins ~140kya in Eurasia, global
MRCA ~490kya → admixture outside Africa
between local and expanding African populations;
DYS44 (introns & microsatellites, Dystrophin gene, Xp21, Ziętkiewicz et al., 2003):
a lineage closest to the tree root, virtually absent from Africa, left Africa before 160kya
→ admixture outside Africa with local populations;
RRM2P4 (Ribonucleotide reductase M2 subunit pseudogene 4, Xq27, Garrigan et al., 2005: Molecular Biology
and Evolution 22:189-192):
clearly rooted in East Asia (gradient centered there), higher nucleotide diversity in nonAfricans, origin ~2mya (H.e expansion?) → admixture between H.s. and H.e.
The X chromosome is different (from mtDNA, Y) → introgression
from non-African archaic population (even Homo erectus)?
Problems and Issues:
[Intermezzo 1] The Phylogeny So Far
●
the X chromosome is “deviant” and points to:
–
a structured (non panmictic) population of
origin in Africa (Xp21.1);
–
admixture with “archaics” outside Africa, after
the expansion (HS571B2, DYS44), even Homo
erectus (RRM2P4)
Consequences:
●
speciation process as opposed to event;
●
non-species status for modern Homo sapiens
versus “archaics” (Homo erectus?);
●
introgression of X chromosomes (also coding
sequences?).
Problems and Issues:
4. Primate Models & the Speciosity of Homo
●
models for extinct Homo: living primates - usually humans & great apes
(esp. Pan) → speciose model for Homo (Tattersall & Mowbray, 2004; Harvati, Frost
& McNulty, 2004)
●
Papio vs Theropithecus (Jolly, 2001; Holliday, 2003; Hunt, 2003):
–
morphologically distinct → usually classified as distinct genera
(Holliday, 2003)
–
divergence ~5mya
–
but: - they hybridize both in captivity (Jolly, 2001) and nature (Hollyday,
Theropithecus gelada
(Gelada)
2003; Jolly, 2001)
- hybrids viable & fertile (Hollyday, 2003; Jolly, 2001)
→ allotaxa: “phylogenetically close, but well-differentiated and diagnosable, geographically
replacing forms whose ranges do not overlap [...] in which characters are clinally distributed”
(Jolly, 2001)
→ botanists: syngameon (Lotsy, 1925; Skelton, 1993)
→ polytypic species (West-Eberhard, 2003; Wolpoff & Caspari, 1997)
Papio hamadryas
(Baboon)
“[a] strict paionin analogy
would therefore argue that
all Homo (sensu stricto) were
interfertile” (Holliday, 2003:659)
Problems and Issues:
[Intermezzo 2] What are species?
●
important difficulties in defining species (like languages?) (West-Eberhard, 2003; Skelton, 1993;
Howard & Berlocher, 1998; Hey, 2001; Tattersall & Mowbray, 2005; Holliday, 2003)
●
“the biotic world is self-evidently 'packaged' into units” (Tattersall & Mowbray, 2005) →
intuition about species but species counts are meaningless (project boundaries on an
intrinsically messy world) (Hey, 2001): splitters vs lumpers
●
plethora of definitions (Hey, 2001 counts 24):
–
biological species concept (BSC) [≡ isolation species concept (ISC)]: “groups of actually or
potentially interbreeding natural populations which are reproductively isolated from other such groups”
(Mayr, 1942, 1963)
–
→ reproduction
phylogenetic species concept (PSC): “irreducible (basal) cluster of organisms, diagnosably distinct
from other such clusters, and within which there is a parental pattern of ancestry and descent” (Cracraft
1989)
→ morphology
●
palaeontology: only (incomplete) fossils, chronospecies
●
allotaxa: same BSC but different PSCs
Problems and Issues:
5. Regional Morphological Continuity
one of the oldest arguments (Weidenreich, 1947) falsifying replacement scenarios
●
natural selection can mimic regional continuity (e.g., skin color, body structure) → most
informative traits must be neutral (Wolpoff & Caspari, 1997; Relethford, 2001; Lewin, 1998)
●
the mandibular foramen (Wolpoff & Caspari, 1997; Relethford, 2003):
–
polymorphic (horizontal-oval vs normal), probably neutral
–
H-O virtually unique to European fossils
→ unequal admixture (Neanderthals low) (Relethford, 2003; Relethford, 2001)
●
skeletal & cultural continuity between H. erectus and
H. sapiens in China (Wu, 2004)
●
regional continuity in the Far East (morphometric analysis
of 45 fossil crania) (Demeter, Manni & Coppens, 2003)
●
transitional forms (peripheral regions: Australia, Czech Rep.), pairwise comparison → dual
ancestry (Wolpoff et al., 2001)
●
oldest European modern H.s. (Peştera cu Oase, Romania), 34-36 kya: a mandible): “mosaic of archaic,
early modern human and possibly Neandertal morphological features” (Trinkaus et al., 2003)
Problems and Issues:
5. Regional Morphological Continuity Abrigo do Lagar Velho
●
discovery (Duarte et al., 1999) in central Portugal, Lapedo Valley:
–
largely complete skeleton of an ~4 years old child, ~24kya
–
morphological hybrid between modern H.s. and Neanderthal →
criticized by Tattersall & Schwartz (1999) (“simply a chunky Gravettian child”)
→ reanalysis (Trinkaus & Zilhão, 2003): the mosaic seems real
●
window of opportunity for hybrids: very short (unequal population
The fossil. From
Duarte et al. (1999)
–
sizes)
cannot asses: the fertility of this hybrid nor its degree of admixture (F1 or later)
–
socially accepted (not a freak, human-animal monstrosity): burial context (Zilhão & Trinkaus, 2003)
–
strongly suggests admixture between modern Homo sapiens and Neanderthals →
falsifies a model with replacement
Note: regional continuity is a regional/local and not global pattern (contra e.g., Lewin, 1998)
“The broader implication of Lagar Velho I is a final rejection of the Late Pleistocene Out-ofAfrica with complete replacement scenario for modern human emergence” (Trinkaus & Zilhão,
2003)
Problems and Issues:
6. Ancient DNA – Neanderthal mtDNA
●
1997: Krings et al. (Cell 90:19-30): 1st Neanderthal mtDNA → difficult (decay, contamination)
●
9 samples: Vindija (Croatia), Engis (Belgium), La Chapelle-aux-Saints (France), El Sidrón (Spain),
Pairwise diffs: modern humans, Neanderthal &
chimps. From Krings et al., 1997
Mezmaiskaya (N.Caucasus)
●
overall: Neanderthal mtDNA is different from living
modern and contemporary early modern humans
(Relethford, 2003; Krings et al., 1997; Lalueza-Fox et al., 2005;
Caramelli et al., 2003; Serre et al., 2004; Ovchinnikov et al., 2000;
Krings et al., 2000; Weaver & Roseman, 2005)
→ taken to
imply different species [e.g. Johansson – this conference]
●
●
population diversity: similar to modern humans
pairwise differences: outside
modern but within subspecies
range (Relethford, 2001)
●
European modern mtDNA not
more similar → admixture of unequal populations (Relethford, 2001, 2003)
Adapted from Relethford, 2001
Problems and Issues:
6. Ancient DNA – How Informative is it?
●
mtDNA sequences are different from all modern sequences: divergence 365-853kya →
Neanderthal lineages failed to survive but:
●
Nordborg (1998, 2004): any single genetic locus (including mtDNA) cannot rule out
Neanderthal contribution simply because population history ≠ locus history
–
can rule out trivial models only (panmixia or no interbreeding) but cannot reject any
more complex demographic model
→ many independent loci required: ~50-100 (Wall, 2000)
●
Adcock et al. (2001): ancient mtDNA from 4 gracile (Lake Mungo) and 6 robust (Kow Swamp)
–
LM4, LM15 & LM55: Holocene; LM3: Pleistocene (62±2kya, redated at 40±2kya by
Bowler et al., 2003):
–
mtDNA outside living gene pool (partially nuclear insert)
KS: Pleistocene-Holocene boundary, morphologically outside modern Australians but
generally agreed to be their ancestors; mtDNA within current range of variation
→ recent selective sweep/genetic drift excluded LM3 mtDNA from current gene pool
●
contested (Cooper et al., 2001) but authors responded credibly (?)
Problems and Issues:
[Intermezzo 3] The Phylogeny So Far
●
the X chromosome: structured population &
Eurasian introgression
●
ancient mtDNA: not informative
●
regional continuity (incl. Abrigo do Lagar Velho):
–
admixture/introgression → against replacement
& species status for modern Homo sapiens
Consequences:
●
speciation process as opposed to event;
●
non-species status for modern Homo sapiens
versus “archaics” (Homo erectus?);
●
introgression from non-African “archaics”.
Problems and Issues:
7. Global Trends
●
seemingly, the fossil record shows some global trends → very controversial
●
increase in brain size (Lee & Wolpoff, 2003): gradualism & continuity vs stasis in some
lineages vs different rates in different regions
–
94 fossils (1.8-0.05mya): cranial capacities → trends in log-log transform vs time
–
support a single evolutionary process
–
incompatible with punctuation
–
same process for earlier and later data
●
gracilization (Wolpoff & Caspari, 1997): much harder to quantify
●
explanations:
–
common selective pressures → parallel evolution
–
gene flow → spread of globally adaptive alleles/combinations of alleles
genetic drift
●
if confirmed, could support a single BSC view of Homo, connected by global gene flow
Problems and Issues:
8. Genetic Structure of Living Populations
The Apportionment of Genetic Diversity
●
modern humans: genetically very uniform (Jobling, Hurles & Tyler-Smith, 2004; Relethford, 2001)
–
●
vs chimps: 3x (X, mtDNA), 7x (Y), 1.5-2x (auto) (Harding & McVean, 2004; Yu et al., 2003)
“orthodoxy”: variation 85% within, 15% between populations - Lewontin, 1972 (Evolutionary
Biology 6:381-398)
mtDNA):
→ confirmed by later studies (avg. autosome ~83-88% vs ~9-15%, exceptions Y &
but the conclusion inferred from data is wrong (Dawkins, 2004; Edwards, 2003, etc.)
●
Rosenberg et al. (2002):
●
Bamshad et al. (2003):
●
gradations in allele freq: ~150 loci enough globally
no. loci predicting pop appartenance: 160 Alu & microsat: ~100% accuracy
Long & Kittles (2003): FST in humans: violation
of hidden assumptions → 0.10 ≤ FST ≤ 0.15 due
primarily to statistical artifact.
→ there is genetic structure in humans
→ not pop-specific alleles but distributional
properties of many ubiquitous alleles
→ “race” is genetically meaningless
Adapted from Templeton, 1998
Horizontal axis: FST
Problems and Issues:
8. Genetic Structure of Living Populations
Its Evolutionary Interpretation
●
usually: taken to support ROA with replacement: (1) higher genetic diversity in Africa +
African rooting of genetic trees & (2) genetic uniformity of modern humans
→ interpreted as: speciation in Africa → subpopulation split (bottleneck) → WO colonization
alternatives: longer African history (ROA) or larger long-term African population size
–
●
larger pop size: supported by ecological/palaeoclimatic models (Relethford, 2001, 2003)
long-term effective population size (Ne): usually
estimated at tens of thousands:
→ bottleneck (speciation and/or migration)
→ or a metapopulation model: made up of transient
populations connected by migration, subject to
extinction and rebirth by colonization, as well as
fluctuations in local size (Harding & McVean, 2004)
- better explanations for “anomalies” (TMRCA
spread, young Y MRCA vs mtDNA, very low FSTs)
Adapted from Harding & McVean, 2004
Problems and Issues:
[Intermezzo 4] The Phylogeny
●
single polytypic BSC Homo (allotaxa)
●
greater long-term African population
●
long-term global gene flow
●
expansion with admixture
●
ROA with replacement: ruled out;
●
must have:
–
allotaxa status of various Homo “species”
–
pervasive global gene flow, allowing:
–
synchronized world-wide trends
–
regional continuity
–
the special role played by Africa
–
expansion(s) out of Africa with admixture
The Class of Plausible Models:
“Mostly Out of Africa”
●
John Relethford: Mostly Out of Africa (2001, 2003):
bi-dimensional classification:
–
mode of transition: speciation vs multiregionalism (coalescence in a gene flow network)
–
spatio-temporal coordinates (location & timing): African-recent vs delocalized
→ main shortcomings:
–
not specific enough
–
lacks backmigration/gene flow into Africa
The Class of Plausible Models:
“Out of Africa Again and Again”
●
Alan Templeton: Out of Africa Again and Again (1998, 2002):
nested cladistic analysis – phylogeographic method (GeoDIS program):
●
starts from the tips of the phylogeny and
incrementally constructs nested clades one
mutational step at a time
●
nested clades with significant geographic
differentiation → explanations based on gene
flow, isolation and expansions
●
3 main migrations out of Africa;
●
1 major out of Asia;
●
prevalent recurrent gene flow.
→ main problem: the inference key
(Kobling, Hurles & Tyler-Smith, 2004)
Other models: Eswaran's “Diffusion
Wave Out of Africa” (2002)
Conclusions
●
Recent Out-of-Africa with replacement: rejected
●
some alternatives
●
Milford Wolpoff (Thorne, Wu, Caspari, etc.): multiregionalism → framework
–
Relethford's MOOA, Templeton's OOAAA, Eswaran's DWOOA – multiregional
even later ROA (e.g., Stringer, 2002) – implicitly multiregional (admit admixture)
Language evolution:
–
●
not constrained by a putative recent “speciation” - gradual, accretionary model most
probable
●
favorable “mutations” (genetic and cultural) spreading across gene flow networks
●
discrete/catastrophic events defining “modern language”
features – improbable (FOXP2)
●
open-mindedness and criticism when considering human
evolutionary models