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NEWS & VIEWS
NATURE|Vol 463|11 February 2010
in the course of their journey have discovered
various superheavy elements, the latest being
ununoctium, with 118 protons. However,
we cannot be certain that this is the heaviest
element. And progress towards setting foot
on the island of stability is slow because of the
painfully low rates, sometimes only one atom
per week, at which these exotic atoms can be
produced.
The more difficult the synthesis of heavier
elements and the production of their isotopes, the more important is the availability of
better information on their properties, either
through direct measurements or by better
theoretical prediction. The accurate knowledge of masses is particularly critical. Einstein’s
mass–energy equivalence relates the mass of an
isotope directly to how strongly its protons and
neutrons are bound; that in turn determines
whether it can exist and its lifetime before it
decays. This is where Block and colleagues1
have achieved a breakthrough — by performing the first direct mass measurement on the
isotopes of a trans-uranium element and by
providing the information needed to build a
bridge to the island of stability. Furthermore,
they have demonstrated that the technique
chosen is indeed the most promising one for
meeting the challenge of determining the
masses of isotopes of superheavy elements.
Block et al. determined the masses of the
isotopes 252No, 253No and 254No of nobelium.
The result achieved for 253No led to a tenfold
improvement in the accuracy of its mass, and
as a consequence, to an improved knowledge of
the masses of all isotopes in the α-decay chain
of which it is a member. An α-decay chain
is a sequence of radioactive decays in which
elements transform into one another by emitting an α-particle. The measurements were
performed with SHIPTRAP, a facility specifically developed for high-precision experiments
using trapped ions of very heavy and superheavy elements.
For the mass determination, powerful Penning-trap mass spectrometry6 was used. In a
Penning trap, charged particles can be confined
and stored in a strong magnetic field under vacuum for long periods of time. The frequency of
the circular, ‘cyclotron’ motion performed by a
trapped ion is connected to its charge, its mass
and the magnetic-field strength. By determining this frequency, it is possible to obtain the
ion’s mass. Such mass measurements can reach
extremely high precision — relative uncertainties of less than one part in a billion have been
achieved for stable ions7. The real challenge in
applying this approach to superheavy elements
is reaching a high enough efficiency in transferring these rare isotopes as ions into the trap.
Here1, SHIPTRAP has set a new record for the
lowest production rate for which a Penning
trap has been successfully used to measure the
mass of an unstable isotope.
Until now, superheavy elements were identified by their indirect connection by α-decay
chains to known elements around uranium.
Penning-trap mass measurements can tie down
loose decay chains that are not yet connected
to known elements. Block and colleagues’ first
direct mass measurements provide firm anchor
points that are much closer to the superheavy
elements than before, in addition to improving
the accuracy of the mass values for all isotopes
in these chains. And such Penning-trap mass
measurements1 may become even more important in the long term. As the island of stability
is approached, the lifetime of the nuclides is
expected to become longer with the addition
of more neutrons. Theory predicts half-lives as
long as minutes to hours, a trend that is supported by experiments. For half-lives this long,
the identification of superheavy elements on the
basis of the radioactive decay of their isotopes
will no longer be feasible. Identifying new superheavies by weighing them in a Penning trap may
then be the only practical approach.
■
Georg Bollen is at the Facility for Rare Isotope
Beams, Michigan State University, Michigan
48824-1321, USA.
e-mail: [email protected]
1. Block, M. et al. Nature 463, 785–788 (2010).
2. Hofmann, S. & Münzenberg, G. Rev. Mod. Phys. 72,
733–767 (2000).
3. Morita, K. et al. J. Phys. Soc. Jpn 73, 2593–2596 (2004).
4. Oganessian, Y. J. Phys. G 34, R165–R242 (2007).
5. Stavsetra, L. et al. Phys. Rev. Lett. 103, 132502 (2009).
6. Blaum, K. Phys. Rep. 425, 1–78 (2006).
7. Redshaw, M., Mount, B. J., Myers, E. G. & Avignone, F. T. III
Phys. Rev. Lett. 102, 212502 (2009).
PALAEONTOLOGY
Decay distorts ancestry
Derek E. G. Briggs
Experiments with simple chordate animals show how decay may make the
resulting fossils seem less evolved. The consequence is to distort evidence
of the evolution of the earliest vertebrates and their precursors.
Just as human corpses become more difficult
to identify as information is lost through decay,
so too do the fossils of our marine ancestors
from back in the Cambrian. For example,
a remarkable diversity of soft-bodied, fishlike fossils, dating to about 525 million years
ago, have been described from Chengjiang in
China. Uncertainties about the nature of these
creatures, however, have fuelled controversies
about their place in the early evolution of chordates, the group that includes all vertebrates
and some closely related invertebrates.
On page 797 of this issue, Sansom and
colleagues1 describe laboratory observations of
the decay of two living forms similar to these
earliest chordates: the lancelet Branchiostoma
(a fish-like invertebrate that has a stiffened
structure called the notochord) and the larva
of the more familiar lamprey. They find that
decomposition of these two creatures always
occurs in more or less the same sequence.
Features of the head, for example, tend to be
Chordates
Vertebrates
Lamprey (juvenile)
Decay
Stem vertebrate
Decay
Stem chordate
Figure 1 | ‘Stem-ward slippage.’ Decomposition and the loss of morphological features have the effect
of making a fossil seem less evolved than the organism was in life, and therefore closer to an ancestral
(stem) position on an evolutionary tree. This simplified version of the chordate tree shows (left to
right) outlines of representatives of the cephalochordates and urochordates, and three branches of the
vertebrate lineage. Sansom and colleagues’ decay experiments1 with juvenile lampreys show that the
progressive loss of features would lead to interpretation of the resultant fossil as a stem vertebrate and
then a stem chordate. (Figure based on Fig. 3 of ref. 1.)
© 2010 Macmillan Publishers Limited. All rights reserved
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NATURE|Vol 463|11 February 2010
lost before those of the trunk,
fossils, even when the original
including the notochord and
morphology is unknown. There
a
muscle blocks.
is a long history of palaeontoloEven more strikingly, the
gists making observations of
appearance of the carcass is transdecay in living organisms in the
formed by decay until it resembles
laboratory or the field in order to
the much simpler morphology
understand what is preserved in
of an ancestral (stem) chordate.
similar fossils7. Ranking tissues
Attributes tend to disappear
according to how easily they
in the opposite order to that in
degrade may allow an estimate
which they evolved, so that only
of how much decay has preceded
b
the more ancestral morpholfossilization. Recognizing levels
ogy remains. As a result, the
of susceptibility to decay may
corresponding fossil falls in a
serve to separate features that
misleadingly low position on the
have been lost through decompochordate evolutionary tree. This
sition from those that were absent
process of ‘stem-ward slippage’,
in the first place, a distinction
as Sansom et al.1 call it, clearly
that may be crucial in constraincompromises the usefulness Figure 2 | Early Cambrian yunnanozoan. a, This specimen is well preserved,
ing the place of a fossil on the
of such fossils in revealing the but, like others from the Chengjiang biota, it is flattened in shale. The
chordate tree.
pattern of early chordate evolu- orientation of the animal in the rock and the amount of decay before
The unfamiliar appearance
fossilization vary between examples. Scale bar, 10 mm. b, A reconstruction
tion (Fig. 1).
of Cambrian creatures may also
of the animal in life.
The fossil record of early
generate another uncertainty
chordates poses a challenge to
— what is the most appropriate
palaeontologists because they predate the details of Chengjiang specimens, for example, living organism for comparison, and how do
evolution of the vertebrate skeleton and are are often preserved as a thin layer of pyrite we equate the different features of the fossil
consequently soft-bodied. Their lack of hard (iron sulphide, often later replaced by iron with those in living animals to determine the
parts means that they fossilize only in extraordi- oxide), which forms as a result of the activity place of the extinct form6? This may boil down
nary circumstances; early chordates have been of sulphate-reducing bacteria5. The infor- to questions as fundamental as whether a linear
discovered in some of the most famous Cam- mation available depends on a race between structure is the gut or the notochord. Sansom
brian fossil deposits such as Chengjiang2, and decay and fossilization. Effects created during and colleagues’ analysis1 offers a cautionary
the Burgess Shale near Field in British Colum- decay and preservation may account for some tale for those who would interpret soft-bodied
bia3. Such fossils are flattened on the surface of of the controversy surrounding the placement fossils, but the result provides no panacea for
the shale and are consequently difficult to inter- of the Chengjiang chordates6. For example, the problem of placing these extinct forms
pret (Fig. 2). They have also undergone some as depicted by the red shading in Figure 1a on the tree of life. We still need to rely on selectdecay — the details that survive depend on just of Sansom and colleagues’ paper1 on page 798, ing the most informative fossils and avoiding
how much decomposition occurred before the bizarre yunnanozoans have been inter- preconceived ideas about the nature of the
fossilization.
preted variously as stem deuterostomes, stem animal and the features of its anatomy.
Ironically, however, microbial decay is often hemichordates, stem chordates, stem cephaloIs stem-ward slippage just an isolated
an essential driver of the preservation process, chordates and stem vertebrates.
palaeontological example of Murphy’s law —
particularly where fossilization involves the
Decay is the norm, and researchers must take in this case, that the most useful evidence is
rapid precipitation of minerals4. Morphological it into account when interpreting soft-bodied least likely to be preserved — relevant only to
COSMOLOGY
Census at a distance
Cosmologists have long been
occupied with trying to understand
how galaxies formed and evolved
into their intricate shapes and
forms. With François Hammer
and colleagues’ results of a
comparative census of galaxy
types at two different epochs in the
Universe’s history, they will become
even busier (R. Delgado-Serrano
et al. Astron. Astrophys. 509, A78;
2010).
In his early-twentieth-century
classification scheme, Edwin
Hubble categorized the galaxies
seen in today’s Universe into three
groups according to their visual
appearance: ellipticals, spirals and
742
lenticulars (the last being an
intermediate type between
the first two). Odd-shaped
assemblies of gas, stars and
dust that don’t fall into any of
these categories are termed
peculiars.
But is the census of today’s
Hubble galaxies similar to
that of moderately distant
galaxies? The conventional
view has been that it is.
Surprisingly, Hammer and
colleagues’ scrutiny of two
samples — 148 galaxies selected
from the Great Observatories
Origins Deep Survey, seen as they
were 6 billion years ago, and 116
local galaxies from the Sloan Digital
Sky Survey — finds that it isn’t.
The authors demonstrate that, in
the distant sample, the fraction of
© 2010 Macmillan Publishers Limited. All rights reserved
peculiar galaxies (pictured) is about
five times higher than that of their
present-day counterparts, and that
spirals were about 2.3 times less
abundant. In striking contrast, the
number of ellipticals and lenticulars
is essentially the same.
But there’s much more to
Hammer and colleagues’ study than
counting galaxies and sorting them
into classes. Their investigation
suggests that many of the distant
peculiar galaxies evolve to become
today’s spiral galaxies through
gas-rich galactic collisions — a
conclusion that is at odds with the
belief that galaxy collisions lead to
the formation of ellipticals. Such
evidence needs to be taken into
account in testing theories of galaxy
formation and evolution.
Ana Lopes
NASA/ESA/SDSS/F. HAMMER & R. DELGADO-SERRANO (OBS. PARIS)
A, D. SIVETER/REF. 2; B, J. CHEN
NEWS & VIEWS
NEWS & VIEWS
NATURE|Vol 463|11 February 2010
early vertebrates? Or is it a more pervasive
phenomenon? In general, the answer is that
stem-ward slippage is widespread: all fossil
animals with a high proportion of missing
information tend to fall out near the base of an
evolutionary tree through the lack of morphological features (such as structures in the head,
in the case of chordates) to ally them with more
evolved groups. And the resulting tree may be
biased unless the decay sequence is random
relative to the tree’s branching order — that
is, the order in which characters evolved. As
well as prompting caution in interpreting
soft-bodied fossils, Sansom and colleagues’
research1 may turn out to be important in
identifying a way to assign confidence limits
to the placement of these extinct forms in the
tree of life.
■
Derek E. G. Briggs is in the Department of
Geology and Geophysics, and the Yale Peabody
Museum of Natural History, Yale University,
New Haven, Connecticut 06520, USA.
e-mail: [email protected]
1. Sansom, R. S., Gabbott, S. E. & Purnell, M. A. Nature 463,
797–800 (2010).
2. Hou, X.-G. et al. The Cambrian Fossils of Chengjiang, China:
The Flowering of Early Animal Life (Blackwell, 2004).
3. Caron, J.-B. & Rudkin, D. (eds) A Burgess Shale Primer:
History, Geology, and Research Highlights (Burgess Shale
Consortium, 2009).
4. Briggs, D. E. G. Annu. Rev. Earth Planet. Sci. 31, 275–301
(2003).
5. Gabbott, S. E., Hou, X., Norry, M. J. & Siveter, D. J. Geology
32, 901–904 (2004).
6. Donoghue, P. C. J. & Purnell, M. A. BioEssays 31, 178–189
(2009).
7. Schäfer, W. Ecology and Palaeoecology of Marine
Environments (Chicago Univ. Press, 1972).
a No benzodiazepines
Glutamatergic
neurons
B1-containing
GABAA receptor
Glutamate
receptor
Dopaminergic
neurons
Interneurons
Inhibition
Low
activity
b Benzodiazepines bound
Glutamatergic
neurons
NEUROSCIENCE
Lack of inhibition leads to abuse
Strengthened
synapse
Benzodiazepine
Arthur C. Riegel and Peter W. Kalivas
Chronic drug use can lead to addiction, which is initiated by specific brain
circuits. The mystery of how one class of drugs, the benzodiazepines,
affects activity in this circuitry has finally been solved.
Common illnesses such as anxiety disorders,
insomnia and even muscle spasms are treated
with benzodiazepine drugs, of which diazepam
(Valium) is perhaps the best known. But
both conventional benzodiazepines and newer
benzo diazepine-like compounds (such as
zolpidem) are addictive. This limits the therapeutic potential of an otherwise safe class of
drugs that has broad clinical applications.
Most addictive substances activate the same
brain circuitry: the dopaminergic system,
which is also stimulated by natural rewards,
such as food and sex. Benzodiazepines
stimulate this circuitry, but the underlying
mechanism was unknown, prompting speculation that benzodiazepine addiction involves
systems distinct from those involved in
dependence on other addictive drugs.
In this issue (page 769), Tan et al.1 report that
the missing link connecting benzodiazepines
to the dopaminergic circuitry is a select group
of GABAA receptors (γ-aminobutyric acid type
A receptors) that reside on neurons known
as inhibitory interneurons. These interneurons coordinate the output of dopaminergic
neurons in the ventral tegmental area (VTA)
of the brain. GABAA receptors are assembled from five subunits, of which there are
several types; the authors find that, in inhibitory interneurons, benzodiazepines increase
the stimulation of GABA A receptors that
contain α1 subunits. This reduces activity in
the interneurons and thereby increases excitability in VTA dopaminergic cells. Through
a series of ingenious experiments, Tan et al.
demonstrate that this mechanism contributes to
benzodiazepine addiction.
In general, the urge to self-administer an
addictive drug is initiated in the VTA by the
‘strengthening’ of excitatory glutamatergic synapses — neural junctions at which glutamate is
the primary neurotransmitter — to dopaminergic neurons. Such strengthening increases
the chance that the synapse will release glutamate and is caused by the recruitment of new
AMPA receptors (a class of glutamate receptor)
to glutamatergic synapses on dopaminergic
neurons. In agreement with an earlier study2,
Tan et al.1 found that benzodiazepines also
cause this effect in mice, regardless of whether
the drugs are injected systemically (whereupon they can interact with GABAA receptors throughout the animal’s body) or directly
infused into the VTA.
The authors obtained similar results with
zolpidem, which binds predominantly to
GABAA receptors that contain α1 subunits. But
they detected no such strengthening in wildtype mice in which a benzodiazepine was coadministered with a compound that blocks the
benzodiazepine binding site on GABAA receptors (a benzodiazepine antagonist), or in mice
that were genetically engineered so that their
α1 subunits were unable to bind benzodiazepines. Taken together, these results showed
that the cellular changes that occur in dopaminergic neurons after a single exposure to benzodiazepines require functional α1 subunits in
GABAA receptors.
Tan et al. then mapped the location of
© 2010 Macmillan Publishers Limited. All rights reserved
Interneurons
Disinhibition
Dopaminergic
neurons
Increased
activity
Figure 1 | Proposed mechanism of addiction of
benzodiazepine drugs. Tan et al.1 report that
benzodiazepine drugs bind to and activate certain
GABAA receptors (γ-aminobutyric acid type A
receptors) — those that contain α1 subunits —
on interneuron cells in the ventral tegmental
area (VTA) of the brain. a, These interneurons
ordinarily inhibit the activity of dopaminergic
neurons in the VTA. Forked branches are
synapses (neural junctions). b, Activation of the
GABAA receptors by benzodiazepines reduces
the amount of inhibition (a process known as
disinhibition, red cross), which increases the
activity of the VTA’s dopaminergic neurons.
This increased activity strengthens the VTA’s
glutamatergic synapses (neuronal junctions at
which the primary neurotransmitter is glutamate)
to dopaminergic neurons, making them more
excitable. Such strengthening, caused by an
increase in the number of glutamate receptors on
the dopaminergic neuron, is the cellular response
to natural rewards (such as food), but it also forms
the basis of the addictive properties of drugs.
α1-containing GABAA receptors within the
VTA’s microcircuitry, and found that the
highest density was not on dopaminergic
cells, but on the inhibitory interneurons —
the same class of cells targeted by opioid drugs
such as morphine3. In fact, much of morphine’s addictive potential is attributed to the
inhibition of these interneurons in the VTA.
To better understand commonalities
between benzodiazepines and opioids, the
authors 1 treated α1-mutated mice with
morphine. Surprisingly, they observed that
morphine caused the cellular changes
743