The Mammuthus Moment: How to tell a sloth from another sloth
Xenarthra (sloths, armadillos and anteaters) is one of the more diverse and hard
to phylogenetically place mammalian orders. For example, it is still not
completely clear what other group of mammals are the closest relatives of xenarthrans. This extends to within group comparisons where
sloths alone contain almost 100 distinct genera. Five species contained in two
genera are all that remain of sloths since the end-Pleistocene mammalian mass
extinctions. That is a lot of lost biodiversity. Both surviving groups are
arboreal though many extinct sloths were not. They ranged in size from the
elephant sized Mylodon darwinii found in
Patagonia to bear sized megathirids such as Nothrotheriops shastensis
which were once abundant in the American southwest. For a number of reasons, including
convergent evolution of traits to the sheer number of fossil groups, tracing
the relationships among sloths morphologically has been extremely difficult.
The questions are, which sloths are related to
which other sloths? More importantly for modern biology, which extinct sloths
are the few remaining extant sloths groups related to? Are sloths descended
from a common arboreal ancestor or did arboreality emerge independently in
different lineages? What can molecular biology bring to the table?
Several studies using ancient DNA have been applied to sloths. They have
addressed a variety of questions including taxanomic
but also ecological and dietary such as the determination of sloth diet from
ancient dung (Poinar et al. 1998; Hofreiter
et al. 2000). Braving the low copy DNA, fear of lab contamination, and the
smell of very old poop, these researchers were able to identify the balls of
dung they were analyzing as being sloth in origin and more importantly for
their study, they could identify the plants the sloths had eaten from the DNA
sequences they retrieved. Looking at dung of different ages opened a window
into the diet over the thousands of years in which the climate was drastically
changing and the animals such as giant ground sloths were heading towards
extinction. But it still could not tell us who is who.
Ancient DNA is very limited and you can only examine remains that are
preserved well enough to have a chance of containing trace DNA. In other words,
forget
The first study to address this only included Mylodon, as no
suitable remains from any other group were known at the time. The remains they
used came from a cold cave in
Since this first study, Nothrotheriops
DNA has been extracted from dung (as noted above). Not only that, Mylodon
nuclear DNA was retrieved in the interim. This is novel, as most ancient DNA
studies have relied exclusively on mitochondrial DNA, which only tells you what
happened to the maternal lineage of any given group (Greenwood et al. 1999).
The authors managed to systematically retrieve nuclear DNA sequences from a
variety of extinct mammals including mammoths, sloths and an extinct cave bear.
It was a proof of principle study and did not address any phylogenetic
questions however.
Back again to who is who among sloths. Adding Nothrotheriops to the mix using ancient DNA further
supported the findings of the initial study (Greenwood et al. 2001). This study
combined all the work of the previous phylogenetic attempts and added more DNA
sequences to the mix to attempt to resolve the issue with a greater quantity of
data. An analysis of where the resolution was being lost was also employed
which demonstrated that some genes are evolving at different rates than others
which may have complicated matters somewhat for the earlier studies. In any
case, again, Mylodon clearly associated with arboreal two toed sloths,
while three toed sloths clustered with Nothrotheriops,
completely contradicting the expectations from morphology. It would be like
finding out we are more like gibbons than chimpanzees at the DNA level…ok, that
is a bit of an exaggeration since the time scale is much larger for sloths than
apes, but it does illustrate the contradiction.
Since science does not just say the day is over we are finished, the
work has not stopped and now nuclear DNA phylogenies are coming into play (Poinar et al. 2003). For some odd reason, the authors did
not include Mylodon in the study, though in previous attempts they had.
This of course severely limits the utility, but it’s a good start…and I guess
they can get another paper out of it by publishing Mylodon separately.[T2]
The conclusions from each of these studies are very tentative. Can you
find a testable hypothesis in any or all of these studies, e.g., Mylodon is
more closely related to…what? (Hint, you
can figure it out by reading the online abstracts.) Does all the data
contradict or support the conclusions, i.e., morphologically and molecularly,
do they say the same thing? If not, is there something wrong with one
methodology? Both?
And what should be done next? How would
you resolve the relationship among the extant sloths, Mylodon, and Nothrotheriops? Would the scientists referenced
impede the publishing of results that suggested their conclusions were wrong
and reject morphological work that does not support the DNA evidence? Is
science omitting some kind of other evidence that would resolve the question
poof bang? Think slowly as a toed sloth moves and you might find some answers.
References:
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Nuclear DNA sequences from late Pleistocene megafauna.
Mol Biol Evol. 1999 Nov; 16(11):1466-73.
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A molecular phylogeny of two extinct sloths. Mol Phylogenet
Evol. 2001 Jan; 18(1):94-103. |
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Molecular phylogeny
of the extinct ground sloth Mylodon darwinii. |
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Hofreiter
M, Poinar HN, Spaulding WG, Bauer K, Martin PS, Possnert G, Paabo S. |
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A molecular analysis of ground sloth diet through the last glaciation. |
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Nuclear gene sequences from a late pleistocene
sloth coprolite. Curr Biol. 2003 Jul 1; 13(13):1150-2. |
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Molecular coproscopy: dung and diet of the
extinct ground sloth Nothrotheriops shastensis. Science. 1998 Jul 17; 281(5375):402-6. |
