This was written for a class I took last year on prehistoric human evolution, and as such, it's hardly my opus work. In fact, I almost wanted to edit into oblivion, but I settled for scattered notes in the text to explain myself as I went along.
Those Tricksy Little Hobbitses: the Argument over H. floresiensis
Outline
-
The
Discovery
-
Peter Brown
and his team of Indonesian and Australian researchers found a
fairly-complete H. floresiensis skeleton in a cave called
Liang Bua on the island of Flores; they designated this specimen
‘LB1’.
-
Soon
afterwards, they found a second much less-complete skeleton, called
‘LB2’.
-
Not long
afterwards, they found the scattered remains of seven more
individuals, duly noted as ‘LB3’ through ‘LB9’. Of all the
remains, LB1 was the most complete, and only LB1 had an intact
cranium.
-
H.
floresiensis is an australopithecine
-
Stature (~1
m) and cranial capacity (~320 cm3) are similar to the
earlier australopithecines.
-
The anatomy
of the carpals bears some resemblance to earlier australopithecines
as well. (note: basically, their hands kind looked like australopithecine hands, with
long, curved bones ideal for a lifestyle spent partly in trees, and
partly on the ground, like many researchers believe that the earlier
australopithecines lived)
-
But — if H. floresiensis were an australopithecine, then how
did it wind up all the way in Australasia? (note:
as of right now, most researchers believe that the first hominid to
leave Africa was Homo erectus; however, many are starting to think
that there were earlier waves of migration from the continent, and my
professor told me during office hours that this would become textbook
knowledge in a decade or two --- stay tuned!)
-
H.
floresiensis is a Homo erectus
-
The
thickness and shape of the cranial vault suggest a relationship
between H. floresiensis and H. erectus
-
Humeral
torsion is much lower than that of H. sapiens (at 110°,
where the average for H. sapiens is closer to 130° -
140°); this is more similar to the Nariokotome Boy than the
average for a typical modern specimen of H. sapiens. (note:
this basically means that the upper arm bone, the humerus, showed a
shallower angle between the shoulder joint and the elbow joint.
Assuming you, Dear Reader, are an anatomically-modern Homo sapiens,
your shoulder and elbow should form roughly an obtuse angle when
viewed from above while you're doing your best cadaver impression. With H. floresiensis, the angle was much shallower, almost a right angle.)
-
H.
erectus was definitely able to cross bodies of water (how else
could it get to Java?) (note: many think that H. erectus was capable of making rafts; they certainly used tools. Others think
they drifted in a storm, hanging on for dear life onto a piece of
driftwood and being washed ashore on Flores. The distinction blurs
somewhat when you realize that H. erectus really wasn't likely to be the best of engineers, splendid hunters and travellers though they were.)
-
H.
erectus was the first hominid to definitively leave Africa (though there is mounting evidence to the contrary — stay tuned!)
-
But — (note: I have no idea why I left this blank, but what I meant to say was something along the lines of 'but gosh, gee whiz, H. floresiensis sure don't look like a H. erectus!' A waffle of an answer, to be sure.)
-
H.
floresiensis is a Homo
sapiens with a serious case of secondary
microcephaly
-
Microcephaly
is, simply put, the cranial vault being significantly smaller than
average. Primary microcephaly appears before birth and almost
always results in stillbirths; the most extreme example of this is
called ‘anencephaly’, and results in the complete absence of a
cerebrum or cerebellum. Secondary microcephaly is the failure of an
otherwise-normal brain to reach full adult size, and usually
results in profound mental handicapping.
-
The
encephalization of H. floresiensis is lower than both H.
erectus and H. sapiens (and lies within the lower range
found for australopithecines), even accounting for insular dwarfing
and the allometric effects thereof. (note:
insular dwarfing is where mammals show smaller average body
sizes on islands than they do on mainland areas; the opposite trend is
true for reptiles, which is why you see Komodo
dragons and monitor lizards on islands, but only
tiny British-accented geckos on the mainland)
-
But — Evidence of complex cultural behavior (i.e. cooperative hunting
of Stegodon) (note: a species of elephant) and tool-making make H. floresiensis seem a lot more intelligent than its cranial capacity would
suggest; indeed, the encephalization ratio (note: brain-to-body-size ratio) lies somewhere between
the great apes and H. erectus, both of which show examples
of tool-making and complex cultural behaviors.
-
And
also — analyses of a brain cast of LB1 shows an enlarged
frontal lobe in proportion to the rest of the brain, which is
something entirely anomalous — secondary microcephalics have
more-or-less ‘normal’ cerebral proportions. This may account
for how H. floresiensis could engage in such complex
behaviors and have such a small encephalization quotient. Others
suggest that the microstructure of the brain of H. floresiensis was different, allowing it to engage in complex mental behaviors
without needing quite so much cerebral hardware.
-
H.
floresiensis is something entirely different
-
H.
floresiensis’
wrists show traits primitive to before even the split between African
apes and humans. This is unlikely to have been the result of allometric changes due to their adaptations to a resource-poor
insular environment; also, this is unlikely to be due to some sort of
congenital deformity in the specimens analyzed.
-
H.
floresiensis exhibits a confusing mosaic of traits both
primitive and derived, and there are aspects of its dentition and
postcranial skeleton that are autapomorphic. While may of these
traits can be explained as the allometric effects of insular
dwarfism, some of them (such as the aforementioned dentition)
cannot be quite so easily dismissed.
-
The
paleoanthropological community has agreed that this classification
for H. floresiensis is the most likely one (though the case
is still very much open).
-
H.
floresiensis and the Palau specimens
-
Very
recently, Lee Berger and a team of researchers happened upon a pair
of insular-dwarfed H. sapiens skeletons on the island of
Palau in Indonesia. While they show some traits reminiscent of H.
floresiensis (such as small size and similar cranio-facial
proportions), they also lack some traits found in the Liang Bua
finds (such as iliac flaring and the dental and carpal
abnormalities).
-
The findings
were published in a rush, and further analysis on the finds needs
to be done to better compare the Palau finds to the Flores finds.
As it stands right now, the Palau finds are fairly typical for
island-dwarfed H. sapiens in a tropical environment, and
Berger does not believe that they have any close link to H.
floresiensis.
Discovering Homo floresiensis
Ever since Peter Brown and Indonesian-Australian research team
discovered in 2003 the remains of what would become known as Homo
floresiensis — or the 'Hobbits' — the academic
community has been embroiled in a fierce debate as to just how to place this diminutive hominid in the hominid phylogeny. Some
classed it with the australopithecines, some believed it was a
subspecies of Homo erectus, and some held that the sole
complete cranium ('LB1') of H. floresiensis that was found was
that of a modern Homo sapiens with a serious case of secondary
microcephaly. But whatever H. floresiensis was, it represented
a tantalizing side branch of hominid evolution, co-existing with
modern H. sapiens until very recently in both time and
territory — present estimates based on available fossil evidence
place the extinction of H. floresiensis at a scant 18,000
years ago (Brown 2004), and maybe even as recently as 13,000 years
ago, based on stone tool evidence (Morwood 2004). The H.
sapiens populations on Flores arrived anywhere between 55 to
35 kya, overlapping with H. floresiensis for at least 23 kya,
and most likely even longer than that (Brown 2004).
The discoveries
were made in a cave called Liang Bua, located 25 kilometers from the
coast of Flores. Evidence has been found on the island that indicates
that the island may have been inhabited by hominins since at least
840 kya (Morwood 2004). The cave itself was the final resting place
for one specimen of H. floresiensis (designated specimen
‘LB1’), dated to approximately 18 kya, as well as that of another
very incomplete specimen designated ‘LB2’ (Brown 2004). Later on,
remains of seven other specimens had been recovered in varying
degrees of completeness, designated 'LB3' through 'LB9' (Morwood
2005). The degree of articulation still present in the skeletal
remains of LB1 indicated that it was buried in fine river silt very
soon after death, something consistent with evidence that the cave
was flooded regularly by the nearby Wae Racang river in the past.
Found along with the skeleton were the remains of several small
fauna, including fish, frogs, snakes, tortoises, varanids (including
Komodo dragons), birds, rodents and bats, as well as the skeletal
remains of dwarf Stegodons (a species of elephant) that
inhabited the island. Surprisingly, several of the bones found were
charred, something very unlikely to happen spontaneously on a cave
floor, indicating that H. floresiensis could very well have
had the ability to cook food with fire (Morwood 2004).
Handy
Little Hobbits
Additionally, a
dense concentration of prepared-core stone tools (more than 5,500 per
cubic meter of stratum in one area) was found surrounding the
skeleton, with a date range of between 102.4 kya and 0.6 kya, but it
was unable to be determined with certainty which species manufactured
these artifacts. Very few tools were found in Sector VII, near LB1,
but the greatest majority of the tools were found in Sector IV. Tools
found in older strata were made by a much heavier and cruder
hammering method than those found in newer strata, and the tools
themselves were found in association with juvenile Stegodon remains, suggesting that whatever wielded these tools were engaged
actively in hunting (Morwood 2004).
Was H. floresiensis an australopithecine?
The differences and
similarities that H. floresiensis had between themselves and
other hominid species are many-fold and confusing, and sparked a very
lively debate. H. floresiensis seemed to be a mixture of
features between australopithecines, H. erectus and H.
sapiens. Physical stature suggested that it was an
australopithecine adapted to a tropical climate and an island
environment, as did its endocranial volume (which overlapped with the
lower range of cranial capacities found in Australopithecus,
and was basically equal to Pan troglodytes and some of the
earliest hominids) (Falk 2005). But it lacked the robust masticatory
adaptations seen in australopithecines and paranthropids, and had
substantially reduced prognathism and facial height relative to both
genera. Furthermore, it had smaller post-canine teeth and overall
reduced mandibular and maxillary dimensions. Cranial capacity and
stature alone are insufficient to conclusively classify these
'Hobbits' with Australopithecus.
Similarities
with H. erectus
H. floresiensis had substantial similarities to H. erectus, however, given the
similarities in cranial shape and thickness shared between the two
species. However, the sheer thickness of the cranial vault resulted
in a much-reduced brain volume relative to australopithecines and
early Homo, especially when compared to similar
Plio-Pleistocene hominids with similar external cranial vault
dimensions, and the morphology of the mandibular symphisis was more
like the LH4 A. afarensis than the Zhoukoudian and Sangiran H.
erectus specimens (Brown 2004). H. erectus certainly
had the capability to reach Flores, as it was well-established in
Asia at the earliest horizon of the 'Hobbit' habitation of the
island; furthermore, H. erectus must have had the capability
to cross bodies of water, as some of the first fossils of this
species were found on Java. H. floresiensis had an
encephalization quotient roughly similar to H. erectus,
especially when we account for its small stature.
H.
floresiensis and microcephalic H. sapiens
Indonesian
anthropologist Teuku Jacob and his colleagues claim that H.
floresiensis was an island-dwarfed H. sapiens with
secondary microcephaly, pointing to the evidence exhibited in the
cranium. Indeed, H. floresiensis does show numerous cranial
asymmetries that cannot be explained away as damage sustained during
deposition and excavation (Jacob 2006).
Secondary
microcephaly is simply a failure of the brain to grow to full adult
size after birth, as opposed to primary microcephaly, which is a
medical condition in which the brain is significantly below average
size at birth. Secondary microcephaly would result in some distortion
of craniofacial features, but would result in the brain being
more-or-less of the same proportions of a very young human; this
almost always results in profound mental redardation (Niven 2006).
Others who support
the idea that H. floresiensis was a H. sapiens with
secondary microcephaly (such as R. D. Martin) point to the
proportions of the brain to the rest of the post-cranial skeleton,
which falls outside of the normal range associated with H.
sapiens, in such a way as to be highly unlikely to result from
typical insular dwarfing (Martin 2006).
H.
floresiensis was not a microcephalic H. sapiens
Opponents to the
idea that H. floresiensis was a microcephalic modern human
point out that there are a number of anatomical anomalies, especially
in the dentition (which show the P3 with an enlarged
occlusal surface as compared with H. sapiens) something that
would be rather unlikely with recorded cases of secondary
microcephaly, which do sometimes result in cranial deformation, but
would unlikely result in significant changes in dentition (Argue
2006). There is also the consideration of the complex cultural
behavior associated with H. floresiensis, which included
tool-making and the controlled use of fire. While there are cases
aplenty of modern microcephalic H. sapiens with intelligence
quotients that fall within the normal range, certainly a
microcephalic with an encephalization as tiny as that of H.
floresiensis would almost certainly suffer from profound
mental handicapping. Conroy and Smith suggest that perhaps the
microstructure of the brain of H. floresiensis is such as
to cause it to fall out of the typical range of variation for
primates — a paradox, to be sure, but it is perhaps the only way to
explain the low degree of encephalization found in H. floresiensis and the complex cultural behaviors associated with them. In short,
Conroy and Smith state that 1 cm3 of H. floresiensis
brain would necessarily be somehow 'denser' than 1 cm3 of
brain matter from their nearest hominid relatives (Conroy and Smith
2006). Niven also analyzed scans of the cranial vault of H.
floresiensis, and found that they exhibited an enlarged frontal lobe
in proportion to the rest of the brain, something that would not be
expected in a typical secondary microcephalic (Niven 2006). Paradoxical and contradictory though these findings may be, the
gross features of H. floresiensis' cranium could not simply be
explained away as mere secondary microcephaly; there is a host of
other less-noticeable features that cannot be so dismissed.
Could H. floresiensis be Something Else Entirely?
Furthermore, when
we analyze the available data on H. floresiensis on its own
terms, we come to see a number of surprising facts. For one, the
wrist structure is like neither Australopithecus nor
Homo! Indeed, the three carpal bones analyzed in the hands of
H. floresiensis appear to have traits primitive to before even
the ape-human split (Tocheri 2007). Parsimony would suggest that H.
floresiensis would have been descended from a species that split
off and radiated before the ape-human split, making it
unlikely to be descendants of populations of Australopithecus,
H. erectus or H. sapiens that colonized Flores. It
would be something entirely different, a sentiment that both M. J.
Morwood and Peter Brown agree with. Certainly, the haphazard
smattering of traits exhibited in the skeleton of H. floresiensis suggests that it is something that is neither Australopithecus nor Homo.
This notion
is buttressed somewhat by some evidence in the post-cranial skeleton
of H. floresiensis. The fossils' arms exhibit such a low
degree of humeral torsion as to be more like that of apes and the
earliest ancestors of humans, than they would of more recent hominids
(Larson 2007). The pelvis shows some traits that are seemingly
autapomorphic, such as an iliac flare (Brown 2004). The femur is much
straighter and has less-developed attachment points for the large
muscles of the leg, and both the overall femoral length and high
bicondylar angle fall within the range of Australopithecus and
Pan, even allowing for allometric effects related to the small
size of the specimens (Brown 2004). It would seem that if we
wanted to place H. floresiensis anywhere on the hominid
line, it would make the most sense to place it fairly early on,
certainly before Homo.
It is not beyond
the realm of possibility that hominid populations left Africa before
H. erectus; however, any evidence for such migrations would be
nearly impossible to find, as the most likely routes for migration
would have fallen victim to the slings and arrows of geology and
changing sea levels. Any speculation on this topic is merely that —
pure speculation — until we find more definitive evidence that H.
floresiensis is a representative of a species that predates
even the African ape-human split. So far, the notion that H.
floresiensis predated the ape-human split is supported by an
analysis of only three carpal bones, flimsy evidence at best (but
conclusions of similar magnitude have been drawn on even less
skeletal evidence).
However, there are
other features that can be brought to bear in support of the idea
that H. floresiensis forms its own clade. For one, the
brain itself yields some interesting features. An analysis of the
cranial vault of LB1 shows that it had a very convoluted frontal
lobe, especially in the areas associated with self-awareness and
other higher mental traits more often associated with late Homo (Falk 2005, Niven 2006). Despite the fact that H.
floresiensis had a brain more comparable to chimpanzee than
human being, the encephalization quotient places it somewhere in the
mid-range between the great apes and H. erectus (Falk 2006).
'Hobbit Man' was no mere ape; it might well have been able to fashion
stone tools and engage in cooperative hunting, and then bring the
meat back to cave homes and cook it with fire. The general consensus
in the anthropological community is that H. floresiensis is an
entirely new and separate species of hominid, with a host of
primitive features that place its divergence on the evolutionary
timeline back to perhaps even some time before the split between
African apes and hominids. This is a conclusion I am inclined to
agree with, since the plesiomorphies seen in H. floresiensis seem to be numerous enough and of such a character as to be unlikely
to have resulted from developmental abnormalities or the distortion
in bodily proportions concomitant with island dwarfing.
Other
Dwarfed Hominids
On this token, it is worth noting the very recent discovery by Lee
Berger and his team of researchers of a pair of island-dwarfed H.
sapiens fossils found on the Indonesian island of Palau. It is
worth noting this development, because it gives us a clear picture of
what a H. sapiens might typically look like under island
dwarfing; while it does share some characters in common with H.
floresiensis, such as similar craniofacial dimensions and
especially cranium-to-body ratios, it lacks some features
autapomorphic to H. floresiensis (such as a distinct flare in
the ilium, or some dental traits unique to the specimens uncovered in
Liang Bua) (Berger 2007).
Works Cited
D. Argue, et al.
2006 Homo floresiensis: Microcephalic, pygmoid,
Australopithecus, or Homo? Journal of Human Evolution
51:360-374.
P. Brown, et al.
2004 A new small-bodied hominin from the Late Pleistocene of Flores,
Indonesia. Nature 431:1055-1061. Nature Publishing Group.
Berger, Lee R., et
al.
2007 Small-bodied
humans from Palau, Micronesia. PloS ONE 3(3):e1780.
Conroy, G.C. and
Smith, R.J.
2006 The size of scalable brain components in the human evolutionary
lineage: With a comment on the paradox of Homo floresiensis.
Department of Anatomy and Neurobiology, Washington University
Medical School, St. Louis.
Falk, Dean, et al.
2005 The Brain of LB1, Homo floresiensis. Science
308:242-245.
Dean
Falk, et al.
2006 Response to Comment on “The Brain of LB1, Homo
floresiensis”. Science 312:999c
T. Jacob, et al.
2006 Pygmoid Australomelanesian Homo sapiens skeletal remains
from Liang Bua, Flores: Population affinities and pathological
abnormalities. PNAS 103(36): 13421-13426.
S. G. Larson., et
al.
2007 Homo floresiensis and the evolution of the hominin
shoulder. Journal of Human Evolution 53:718-731.
R.D. Martin, et
al.
2006 Comment on “The Brain of LB1, Homo floresiensis”.
Science 312:999b
M.J.
Morwood, et al.
2004 Archaeology and age of a new hominin from Flores in eastern
Indonesia. Nature 431:1087-1091.
M.J. Morwood, et
al.
2005 Further evidence for small-bodied hominins from the Late
Pleistocene of Flores, Indonesia. Nature 437:1012-1017
Niven, Jeremy E.
2006 Brains, islands and evolution: breaking all the rules.
Smithsonian Tropical Research Institute, Apartado
0843-03092, Balboa, Ancón, Panamá, República de
Panamá.
Tocheri, Matthew
W., et al.
2007 The Primitive Wrist of H. floresiensis and its
Implications for Hominin Evolution. Science
317:1743-45.