Australia: The Land Where Time Began |
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WLH 50 Ancestry nDNA
The
Denisovan mtDNA clade is a sister group to the mtDNA clades of
Neanderthals and recent/living humans (Meyer et
al., 2014). Though on its own
mtDNA is not necessarily sufficient to examine hypotheses of
relationship above the population level (Ballard & Whitlock, 2004;
Eyre-Walker, 2006). It has seemed inexplicable that the Sima de los
Huesos mtDNA is on the Denisovan mtDNA clade and not the Neanderthal
mtDNA clade as the abundant Sima de los Huesos skeletal remains share
features with later European Neanderthals that are uniquely common in
the Neanderthal sample (Arsuaga et
al., 1997; Martinón-Torres et
al., 2012). One of the
ancestors of the later Neanderthals is unquestionably the Sima de los
Huesos hominids. Different phylogenies are given by nDNA and mtDNA for
the same specimens because of their different patterns of evolution;
mtDNA evolves with branching and extinction of lineages as the entire
mtDNA genome is inherited as a whole, while nDNA both branches and
reticulates with the result that gene flow is a powerful mechanism that
alters the structure of the relationships of populations. Besides, mtDNA
of Denisovans, like the Neanderthal anatomical form, can no longer be
found.
When the analysis of the nDNA from the Denison Cave Sample was compared
with the analysis of the mtDNA of Neanderthals it showed that the
Denisovans and Neanderthals together are a sister group to recent and
living humans (Prüfer et al.,
2014). The implications of nDNA studies to date were summarised by Hawks
(2013):
“The pattern of variation outside Africa appears to reflect
interbreeding among populations that were much more separate during the
period before 100,000 years ago, including the Neanderthals and
Denisovans. This historical pattern is not uncommon among mammals (Hawks
& Cochran, 2006), for which incompatibility has rarely evolved in a
period shorter than 1million years (Holliday, 2006). … the recurrence of
the pattern within and outside Africa and the geographic specificity of
Denisovan and Neanderthal descendants both show that interbreeding among
these ancient people occurred within
their habitats. Neanderthals and Denisovans were part of the
biological species
Homo sapiens. Today’s
people around the world are a relict mixture of populations from an
ancient species much more genetically and morphologically diverse than
now.”
Wolpoff & Lee agree with this appraisal (Wolpoff et
al., 1994. A fundamental
difference remains between Denisovans and Neanderthals, when it is
accepted that Denisovans and Neanderthals are sister groups within the
species
Homo sapiens. It has not
been identified within the fossil record who the Denisovans are (Wolpoff
et al., 2014). Fossils cannot
be recognised as Denisovans, and it is also possible that there never
was a recognisable sample of Denisovans in the sense there is a
recognisable sample of European Neanderthals, though Wolpoff & Lee
suggest there are almost certainly Denisovan fossils that are known to
paleoanthropologists at the present. Reasons for this are discussed
below. In the view of Wolpoff & Lee none of the skeletal remains from
the Late Pleistocene (<50 ka: Prüfer et
al., 2014) hominids from
Denisova Cave have diagnostic anatomy. From the range of modern
human populations that contain detectable frequencies of Denisovan nDNA
, that are almost invariably from Island Southeast Asia, Australia and
Oceania (Reich et al., 2011),
where the frequencies of Denisovan genomes are between 1 % and 6%.
The problem is described by Wolpoff & Lee as there being no disparity
between the phylogenies that describe Neanderthal mtDNA and their nDNA
relationships, as being evidenced by the fact that even the toe phalanx
from Denisova Cave with mtDNA close to that of European Neanderthals
also has a nuclear genome that resembles closely that of European
Neanderthals (Prüfer et al.,
2014; Skoglund et al., 2014).
According to Wolpoff & Lee this is not necessarily because the mtDNA
sand the nDNA relationships differ fundamentally in Neanderthals and
Denisovans, but because the groups themselves, Neanderthals and
Denisovans were historically defined quite differently. Neanderthals
have been identified by their skeletal anatomy for more than 150 years
as expressed in Western and Central Europe. The diagnosis of
Neanderthals near the edge of their range is not always clear, as has
been demonstrated by discussions about specimens from the Levant (Amud,
Tabun), North Africa (Jebel Irhoud 1, 2) and Central Asia (Teshik Tash).
Some authors have diagnosed each of these examples as Neanderthal,
though somewhat different from Neanderthals by others. Though in Central
and Western Europe this is rarely the case, where the identification as
Neanderthal has been unambiguous and without controversy, in most part.
After these anatomical identifications have been made, the mtDNA that
was subsequently recovered so far from European and Asian (anatomically
identified) Neanderthals are branches of a single mtDNA clade.
On the other hand Denisovans have not been described anatomically. These
phylogenetic relationships have been based on mtDNA alone, portrayed by
the position of a hand phalanx and 2 molars from the Denisova Cave in
Siberia on a mtDNA tree (Krause et
al., 2010). But more complex
phylogenetic relationships are shown by the subsequent high-coverage
nDNA sequence (Meyer et al.,
2012). And the relationship of specimens with Denisovan mtDNA to
Neanderthals on the tree of mtDNA relationships (Meyer et
al., 2014) differs from the
relationships that anatomy suggests when Sima de los Huesos is also
considered. It is indicated that Sima de los Huesos is 1of the ancestors
of European Neanderthals, as noted above, but not necessarily of all
living humans (Arsuaga et al.,
1997; Martinón-Torres et al.,
2012). It is not known whether the fossil specimens with Denisovan mtDNA
and anatomically similar to each other, in the sense that European
Neanderthals are (Wolpoff, 2014).
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |