Download Consider the overall pattern of hominin evolution, using

Consider the overall pattern of hominin
evolution, using examples from at least two
distinct time periods. What major patterns of
diversity are evident in the hominin fossil
record, and how can they be illustrated using
examples from your chosen period? What do
you consider to be the likely evolutionary and
adaptive processes underlying them?
Word Count: 2,624
Different interpretations of fossil evidence from various periods of hominin evolution yield contrasting conclusions
about how speciose, allopatric and homologous the hominin species were. The mechanisms driving diversity can be
extrinsic (e.g. multiple species in the same ecological space) or intrinsic (e.g. emergence of an adaptive novelty).
These mechanisms promote the improved, and/or novel, exploitation of resources by a taxon. Homoplasies and
hybridisation have been shown to confound phylogenetic reconstructions, as taxa appear more closely related than
they actually are due to morphological similarities. Two periods in hominin evolution will be called upon to illustrate
patterns of diversity and the underlying evolutionary and adaptive processes of these patterns. The first is the
transition between the Pliocene and the Pleistocene epochs (~3-2mya), ending at the emergence of Paranthropus
robustus. The second period is the Upper Pleistocene (~126-11kya), which includes the first global colonisation by
Homo sapiens. Both these periods demonstrate high degrees of hominin diversity in which species may have been
sympatric, leading to competition and interspecies reticulation in areas of ecological overlap.
MAJOR PATTERNS OF DIVERSITY IN THE HOMININ FOSSIL RECORD
How bushy is the hominin tree?
The discovery of new fossil evidence in recent years (Brown et al. 2005; Asfaw et al. 1999; Krause et al. 2010; Berger
et al. 2010) has impacted the interpretation of hominin diversity. While the general academic consensus is that there
is some level of diversity in the hominin lineage (Strait and Wood 1999; Strait 2013), other authors, such as White
(2003), disagree. These differences in opinion are in part due to different taxonomic assignments of fossils that alter
the evolutionary sequence of species (Wood 2010). These views conflict as a result of different approaches to the
promotion of monophyly (Cela-Conde and Ayala 2003). The two main approaches can most simply be divided into a)
reductionist ‘lumping’ of species, or b) or ‘splitting’, resulting in a more speciose, or ‘bushy’, phylogeny (LaPorte
2005). While the total number of lineages is under debate (Wood and Lonergan 2008), the recent consensus is that
hominin evolution is the result of multiple evolving lineages (Hunt 2003), producing the largely speciose, or ‘bushy’,
phylogeny characteristic of the majority of contemporary understandings of hominin evolution (ibid.). This view
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contrasts with the ‘single-species hypothesis’ (Brace 1967), which was a favoured view among some
palaeoanthropologists in previous decades (Hunt 2003).
The effect of different phylogenetic interpretations on the number of evolving lineages is well-exemplified in the
Plio-Pleistocene transition. It is suggested by many that the Australopithecus lineage diversified both
morphologically (to produce three separate genera: Australopithecus, Paranthropus and Homo (Wood and Lonergan
2008)), and spatially (with Australopithecus africanus as the first species attributed to a southern Africa dispersal
(Foley 2013)). Additionally, the early Pleistocene may contain the highest degree of hominin diversity of any time
period (Foley 1989). However, differences in opinion between the ‘lumpers’ and ‘splitters’ can either result in the
australopithecines comprising: a) all of the non-Homo species, or b) a few select species, with Homo and
Paranthropus as distinct genera (Wood 2010; Foley 2013). The conclusion is largely confounded as the extents of
monophyly of Paranthropus and early Homo have yet to be established (Foley 2002). The unique morphology shared
by all members of Paranthropus supports the legitimacy of this genus. However, as will be further outlined,
homoplasy and hybridisation complicates palaeoanthropology’s reliance on fossil evidence for this period.
Evidence in the Upper Pleistocene also polarises academic opinion. Two conflicting models, the ‘Multiregional’
hypothesis and the ‘Out of Africa’ hypothesis, provide explanations for the origin of Homo sapiens. Multiregionalism,
in its most recent form (Wolpoff 2000), posits that H. sapiens are polytypic, evolving in multiple locations around
Eurasia, retaining morphological similarities from the archaic species of previous dispersals (Caramelli et al. 2003;
Raghavan et al. 2009). The evidence for this hypothesis relies on the continuity of morphological similarities of Homo
erectus and H. sapiens populations in regions of Eurasia, particularly East Asia (ibid.). The conflicting model, ‘Out of
Africa’, suggests that H. sapiens evolved from a single common ancestor in Africa (Caramelli et al. 2003). Currently,
the oldest evidence for H. sapiens is found in Ethiopia (Omo Kibish, McDougall et al. 2005). Also found in Africa are
cranial fossils that suggest the existence of hominins more archaic than H. sapiens, but which are not anatomically
similar enough to Homo heidelbergensis or the Neandertals (Homo neanderthalensis) to be attributed to either
taxon (e.g. at Jebel Irhoud, Laetoli 18, and Florisbad, Wood 2010). Finally, African H. sapiens populations exhibit the
greatest diversity of mtDNA, suggesting that these populations have had the longest times to accumulate diversity
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via mutation (Lahr and Foley 1994) Therefore, multiregionalism and ‘Out of Africa’ can support different numbers of
lineages in the Upper Pleistocene. Despite this, however, synchronicity of other, morphologically distinct species to
Homo sapiens (e.g. Neandertals) (Wood 2010) supports the assertion that there is more than one separate evolving
lineage at this point in hominin evolution.
Were hominin species ever sympatric?
While Foley (1989) claims that during these more diverse periods of hominin evolution, there is sufficient evidence
to suggest that there were multiple synchronic species. There is, however, insufficient fossil evidence to determine
the extent of ecological overlap and sympatry (ibid.). An indicator of sympatry is interbreeding between closely
related species; though this does not fully describe the effect of an ecological overlap.
Howell (1978 in Foley 1989) considered the extent to which hominin sympatry occurred in the Plio-Pleistocene and
identified that at multiple sites of an indeterminate species of Homo (likely Homo habilis), fossils attributed to a
‘robust’ australopithecine (Paranthropus) were uncovered in the same stratum as the Homo fossils. Additionally,
Paranthropus boisei fossils have been attributed to the same stratum as A. africanus (Foley 1989). Therefore, Foley
(1989) suggests that multiple species of hominin, from multiple genera, may have been sympatric, rather than just
having coexisted in the ‘same biogeographical region’. Additionally, a new discovery of P. boisei at Malema in Malawi
suggests that P. boisei and Paranthropus robustus may have been sympatric as the distance between sites to which
each species has been associated has been significantly reduced (Holliday 2003). Holliday (2003) consequently
suggests that some degree of hybridisation may have occurred given the possible ecological overlap. Similarly,
earlier dates attributed to A. africanus at Sterkfontein in South Africa, could support hybridisation between A.
africanus and P. boisei, with the resultant hybrid P. robustus (ibid.). Unfortunately, the process of hybridisation has
not been fully understood with respect to the resultant phenotype, particularly over multiple generations and
including introgression, and therefore, identifying fossil hybrids in the fossil record is problematic (Ackermann et al.
2006).
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In more recent evolution, a range of evidence has been put forward to support sympatry of species in the Upper
Pleistocene (Holliday 2003). While some mtDNA analyses indicate no interbreeding between Neandertals and H.
sapiens (Krings et al. 2000), more recent analyses of ancient genomes do provide evidence of interbreeding between
the two species, as well as between the Denisovans and H. sapiens (Stewart and Stringer 2012). Additionally, limited
morphological evidence also suggests hybridisation. A child skeleton (Lagar Velho I, Duarte et al. 1999), associated
with Gravettian tools, demonstrates a mosaic of traits from both Neandertals and H. sapiens (Holliday 2003).
Tattersall and Schwartz (1999) do not agree with the interpretation of the fossils, postulating that the effect of
introgression would cause hybrid phenotypes to stabilize to one of the parental phenotypes via genetic drift over
multiple generations (e.g. F200, Holliday 2003). However, Holliday (2003) emphasises that the extinction of
Neandertals does not necessitate that all Neandertals traits are neutral or maladaptive, and that the persistence of
traits either via ‘stochastic processes’ or via their ability to provide an ‘adaptive advantage’ to the hybrids is a
theoretically sound assertion.
Are shared morphological similarities between taxa homologous or homoplastic?
In cladistics, shared traits between sister species suggests the inheritance of a trait from a common ancestor
(homology). However, homoplasy (“shared characters not inherited from the most recent common ancestor of the
taxa that express them” (Wood 2010: 8908)) can significantly confound phylogenetic reconstructions as it causes
two taxa appear more closely related than they actually are (Wood 2010).
‘Paranthropus’ as a valid genus is influenced by the interpretation of megadontic adaptations as homologous or
homoplastic (Ibid.). As megadontia is exhibited to some extent in all australopithecines, the extreme form in
Paranthropus may be explained by convergent evolution (Foley 2002). However, the reintroduction of
‘Paranthropus’ as a new genus implies a monophyletic origin for all species in the taxon (Wood 2010) but many of
the diagnostic features used to assign members to it rely on craniofacial morphology, particularly megadontia and its
associated reinforcements of the facial skeleton (Wood and Lonergan 2008; Wood 2010). If the increased
megadontia in Paranthropus is not from a recent common ancestor, but appeared independently in eastern and
southern African species, then the justification for ‘Paranthropus’ is negated (ibid.). The problem of identifying
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homoplasy is a particular issue for the phylogenetic reconstruction of early hominin evolution due to a reliance on
morphology from fossil hominins.
Understanding later hominin evolution, such as in the Upper Pleistocene, however, is less reliant on morphological
indicators of relatedness. This is particularly true in recent years due to advances in palaeogenomics (e.g. Caramelli
et al. 2003). Nevertheless, disagreements still arise about the origin of similarities, particularly between closely
related populations, such as H. sapiens across Eurasia. Revisions of the ‘Multiregional’ hypothesis have accounted for
the similarities between all humans due to gene flow (Wolpoff et al. 2000). Regardless, genetic evidence strongly
suggests H. sapiens to be of African origin (Relethford 2008). The wider range of evidence in support of ‘Out of
Africa’ suggests that the phenotype of H. sapiens is homologous, rather than homoplastic.
EVOLUTIONARY AND ADAPTIVE PROCESSES UNDERLYING THE MAJOR PATTERNS OF DIVERSITY IN THE HOMININ
FOSSIL RECORD
Morphological and Behavioural Adaptations
When a new species evolves from an ancestral species, it does this via increased, differential survival of a proportion
of the population, resulting in a reduction in reticulation between the populations, causing speciation (Foley 2013).
Some, such as Foley (2002), view evolution as a series of ‘grades’ (as opposed to ‘clades’, Wood and Lonergan 2008)
caused by ‘adaptive radiations’. Therefore, hominin evolution is instead viewed as an accretion of adaptive
radiations, occurring due to differential degrees of ‘allopatry, local adaptation, and genetic drift’ (Foley 2002).
Radiations trigger adaptive responses, resulting simply in diversification, rather than speciation (ibid.). An ‘adaptive
novelty’ is the morphological or behavioural change that provides the capacity to improve or extend the exploitation
of habitats (ibid.). The evolution of an adaptive novelty may break down constraints to expansion that previously
limited the distribution and/or range of populations (Foley 2002). As outlined by Ackermann and Cheverud (2004)
not all morphological changes are necessarily adaptive as they may emerge via other evolutionary processes.
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The ‘adaptive’ radiation of megadontic specialists suggest a morphological advantage to ingesting more coarse and
fibrous plant-based foods than earlier hominins (Foley 2002). These hominins also appear to be more ‘savannadwelling’ than previous species (ibid.). However, Foley’s (1989) explanation of the paranthropine phenotype is
heavily adaptionist. The linear relationship between increasing aridity and speciation is challenged by assertions that
the craniofacial morphology of hominins in the Plio-Pleistocene may have formed by random forces such as drift,
rather than selection (Ackermann and Cheverud 2004). However, the quantitative analysis lead Ackermann and
Cheverud (2004) to conclude that the paranthropine phenotype is the result of nonrandom processes. Furthermore,
the phenotype of Paranthropus is not only a possible autapomorphy of the paranthropines, but perhaps also the
catalyst for the radiation. Contrastingly, however, facial diversity of Homo (excluding Neandertals) is most likely due
to random processes, such as drift (Ackermann and Cheverud 2004).
Foley (2002) suggests that the initial dispersals of H. sapiens may be due to the ability to exploit maritime resources.
Furthermore, it may be that the southern pattern of early dispersals are linked to the supposed coastal adaptations
involved (ibid.). This radiation differs from others as it comprises considerably less ‘morphological diversification’
and includes the ‘first complete global colonisation’ (ibid.). Ackerman and Cheverud (2004) suggest that the
craniofacial diversity of Homo, and in particular Homo sapiens, is influenced by random processes that indicate a
higher reliance on culture. This assertion is supported by Stewart and Stringer (2012: 1320), stating that “Homo was
particularly well disposed to dispersals and eventual differentiation”. Furthermore, it may be that quick adaptability
via culture predisposed H. sapiens to better survive in the variable environment of the Upper Pleistocene, even in
comparison to species that are considered to be better adapted morphologically, such as the Neandertals (Stewart
and Stringer 2012).
Coevolution of Sympatric Species
Fossils providing evidence of sympatric species over a great temporal and geographical range are significant as they
indicate more than one adaptive strategy and therefore multiple ecological niches (Foley 1989). One can therefore
deduce that either not all hominins were challenged by the same ecological difficulties, or that they were and there
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were multiple adaptive strategies that could be utilised (ibid.). Furthermore, the possibility that these sympatric
species may have interbred has implications for our understanding of hominin evolution.
An example of interspecific reticulation that may have occurred in the Plio-Pleistocene is between P. boisei and P.
robustus (Holliday 2003). Interbreeding may have occurred between the geographical boundaries of the parapatric
populations in eastern and southern Africa (ibid.). Similarly to homoplasy, introgression influences the interpretation
of the evolutionary relatedness between the interbreeding species, as they appear more closely related than they
actually are (ibid.). Hybridisation, therefore, can reconcile the homoplasies in Paranthropus (if they are indeed
homoplasies) as it increases the number of homoplasies that could be reasonably subsumed under convergent
evolution (ibid.). A second way in which hybridisation could be used to explain (and reconcile) issues of polyphyly in
the “robust” australopithecines is that ‘Australopithecus’ robustus could be a hybrid between A. africanus and
‘Australopithecus’ boisei (ibid.). However, this interpretation of the fossil record would require synchonicity between
the two species, something which some date boundaries do not permit (Holliday 2003). Nevertheless, should the
latest dates of the A. africanus fossils be correct, these two closely related species could have interbred in the areas
of ecological overlap (ibid.).
Evidence suggests that the dispersal of H. sapiens in Europe was a catalyst in the subsequent extinction of
Neandertals (Stewart and Stringer 2012). Therefore, there was something in the ecological interaction of these
species that influenced this reduction in diversity. However evidence also suggests that interactions between
sympatric populations were more dynamic than just competing for resources. Genomic evidence suggests complex
interbreeding of H. sapiens and Neandertals between the time of H. sapiens’ initial dispersals from Africa and the
Neandertals’ extinction. Furthermore, evidence of interspecific reticulations confirms the inadequacy of the
‘biological species’ concept in hominin evolution.
CONCLUSION
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Thus far, the (tentative) identification of fossil hybrids has been limited. The impact of hybridisation and homoplasies
in confounding phylogenetic reconstructions suggests an overly heavy reliance on the morphology of fossil hominins,
given palaeoanthropology’s current inability to accurately identify homoplastic traits and hybrid fossils. Later
periods, such as the Upper Pleistocene, that yield genetic evidence, may allow for a more reliable conclusion of
hominin relatedness. However, even in the Upper Pleistocene, genetic evidence is limited, and it is currently not
attainable for analyses of earlier periods. This author suggests a more unified approach of morphology and genetics
to the study of hybrids in appropriate periods. These studies may inform our understanding of the resultant
morphology that can then be used in earlier periods where genetic data cannot be extracted. Additionally,
primatological studies of hybrids of closely related extant species (e.g. Ackermann et al. 2006) may prove
instrumental in our future understanding of the hybrid phenotype.
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