A new study, published in the
journal Science, has provided support
for the ‘accretion’ model of Neanderthal evolution. ‘Classic’ Neanderthals,
i.e. humans possessing the full suite of Neanderthal characteristics, do not
appear in the fossil record until 130,000 years ago. However, French palaeoanthropologist
Jean-Jacques Hublin has proposed that Neanderthal characteristics appeared
gradually over time, in a piecemeal fashion.
Thus, for example if Feature X
appeared in one population and Feature Y in another, then interbreeding between
the two populations would have resulted in a population possessing both Features
X and Y. Over time, populations gradually acquired the full suite of
Neanderthal characteristics by a process of accretion, resulting in a gradual
transition from Homo heidelbergensis to
Neanderthal. The accretion model explains ‘proto-Neanderthal’ features seen on
certain fossils dating to the period prior to the appearance of the ‘classic’
Neanderthals. These include a 400,000-year-old fragmentary skull from Swanscombe
in England and the 225,000-year-old Steinheim skull from Stuttgart, Germany.
Much of the evidence we have
regarding Neanderthal origins comes from a single site in the Sierra de
Atapuerca of northern Spain, near the city of Burgos: a Middle Pleistocene
human burial pit known as Sima de los Huesos. The name translates – rather
appropriately – as ‘the Pit of Bones’. Sima de los Huesos is a small muddy
chamber lying at the bottom of a 13 m (43 ft.) chimney, lying deep within the Cueva
Mayor system of caves. Investigation of the site has proved to be long and
difficult. The most immediate problems are logistical. The cramped site is
located more than 500 m (⅓
mile) from the mouth of the Cueva Mayor and is hard to access, necessitating at
times crawling on the stomach. Another problem is the disturbance to the site
caused by the many generations of souvenir and fossil hunters. Systematic
excavation commenced in 1984 and has continued ever since. To date, over 2,000
fragmentary hominin fossils have been recovered, including three skulls. In
total, the remains are thought to represent at least 32 individuals of both
sexes. It is likely that the site was simply used for the hygienic disposal of
the dead, because there is no evidence to suppose that any of the individuals
were deliberately killed and the bones show no sign of injuries caused by
spears or clubs.
Study of this enormous collection
of bones is still in progress, and is likely to continue for some time yet as
the site yields further fossils. However, it has become clear that the fossils
show a mixture of Homo heidelbergensis and
Neanderthal characteristics, just as would be expected if the accretion model
is correct. The key question is how old is the site? Uranium series dates
obtained in 2007 suggested that they were at least 530,000 years old, making
the Sima people older than some Homo
heidelbergensis remains from southern Europe and the Balkan region that show
no incipient Neanderthal characteristic features.
The new study considered 17 crania,
including seven new specimens. The sample shows a consistent morphological
pattern with derived Neanderthal features present in the face and anterior of
the cranial vault, many of which are adaptations to aid chewing of food. This
suggests that facial modification was the first step in the evolution of the
Neanderthal lineage, consistent with the accretion model evolution, with
different anatomical features evolving at different rates.
The researchers also used a variety
of techniques including combined electron spin resonance/uranium series to
obtain a revised date of 430,000 years old, which gives a far better fit with
the accretion model.
References:
1. Arsuaga, J. et al.,
Neandertal roots: Cranial and chronological evidence from Sima de los Huesos. Science
344 (6190), 1358-1363 (2014).
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