Ardipithecus ramidus Revealed
Special
Online Collection
Genetic data confirm that our closest
living primate relative is the chimpanzee. We shared and
evolved from a common ancestor some 6 million or more years
ago. But what did that common ancestor look like and how did
it live? Hominid fossils predating Australopithecus
-- whose best-known representative is the
3.2-million-year-old skeleton "Lucy" -- have been sparse and
fragmentary, leaving many unanswered questions about the
evolution of our lineage. Now, in a special issue of
Science published on 2 Oct 2009, an international
multidisciplinary team has presented the oldest known
fossils of a potential human ancestor: 4.4-million-year-old
Ardipithecus ramidus from the Afar Rift region of
northeastern Ethiopia. The fossils represent 36 or more
individuals including much of the skull, pelvis, lower arms,
and feet from one female, nicknamed "Ardi." Detailed
anatomical analyses reveal that she stood about 1.2 meters
tall and weighed about 50 kilograms and that her species was
at home both moving along trees on its palms and walking
upright on the ground on feet equipped with an opposable big
toe. Analysis of the hosting rocks and thousands of
fossilized plants and animals indicate that Ardipithecus
lived and ate in woodlands, not open grasslands. One of the
surprising conclusions from this vast body of work is that
chimpanzees have specialized substantially since our last
common ancestor and thus are poor models for that ancestor
and for understanding human innovations such as our ability
to walk. The News Focus section offered further analysis of
the discoveries and a portrait of the grueling and
meticulous field research behind them. A podcast segment and
a video presentation also provided insight into the work and
its significance.
Extra-Ancient Amber
Amber is fossilized plant resin -- typically produced by
trees in response to an injury -- known for its golden
luster and mineral-like qualities. Most amber is Mesozoic or
Cenozoic in age -- dating back as far as 250 million years
ago -- and the most common class is produced primarily by
flowering plants (angiosperms). In a Report in the 2 Oct
2009 Science,
Bray and Anderson reported the discovery of amber
from Carboniferous coal deposits in Illinois dating to ~320
million years ago. The team used a technique called
pyrolysis-gas chromatography-mass spectrometry to analyze
the amber's chemical composition. Surprisingly, they found
that it has a chemistry similar to that of amber from
angiosperms, which evolved almost 200 million years later.
This suggests that the biosynthetic mechanism for producing
complex ambers evolved long before the appearance of
flowering plants. An accompanying
Perspective by D. Grimaldi highlighted the Report,
noting that the discovery presents a cautionary message to
researchers tempted to assign the botanical origins of an
amber deposit to a genus or family of plants based solely on
the amber chemistry.
Carbene Chemistry
Carbenes are highly reactive carbon compounds that feature a
neutral divalent carbon atom with two nonbonding electrons.
In the late 1980s and early 1990s, methods were developed to
stabilize these molecules, transforming them from laboratory
curiosities to versatile catalysts for organic reactions.
Two Reports in the 23 Oct 2009 Science described
advances in carbene chemistry that stretch the versatility
of these molecules even further.
Lavallo and Grubbs showed that in addition to being
useful organocatalysts, a class of stable carbenes called
N-heterocylic carbenes (NHCs) can also catalyze
organometallic transformations. Specifically, the carbenes
induce coupling of monomeric iron olefin complexes to form
clusters incorporating three or four bonded iron centers.
Initial coordination of carbene to iron may facilitate
formation of an iron-iron bond with a second complex. In a
second study,
Aldeco-Perez et al. described a strategy for creating an
NHC isomer in which the divalent carbon is shifted so that
it lies next to only on nitrogen atom, not two. The compound
is stable at room temperature, both in the solid state and
in solution, and can form complexes with gold and carbon
dioxide. A related
Perspective by M. Albrecht noted that "[b]oth studies
open up new and exciting opportunities in synthesis."
Caught in Translation
High-resolution structures of the ribosome have greatly
furthered our understanding of ribosome function, but
certain details about protein synthesis remain unclear. In
the 30 Oct 2009 Science, two Research Articles shed
light on the processes of decoding and translocation (see
the related
Perspective by A. Liljas). To synthesize new proteins,
the ribosome translates the information encoded in messenger
RNA (mRNA) into a chain of amino acids. New amino acids are
delivered by a transfer RNA (tRNA) called aminoacyl-tRNA
with the help of elongation factor Tu (EF-Tu), a guanosine
triphosphatase (GTPase), in a process known as decoding. In
the so-called decoding center, the ribosome ensures that the
codon (a sequence of three nucleotides that specifies an
amino acid) of the mRNA matches the anticodon of the tRNA
and that the correct amino acid is thus inserted.
Schmeing et al. presented the crystal structure of the
bacterial ribosome complexed with EF-Tu and aminoacyl-tRNA.
The structure reveals details of the tRNA distortion that
allows aminoacyl-tRNA to interact simultaneously with the
decoding center of the ribosome and EF-Tu at its binding
site. Elongation factor G (EF-G), also a GTPase, plays a
crucial role in the translocation of transfer RNAs (tRNAs)
and messenger RNA (mRNA) through the ribosome as the
polypeptide chain grows.
Gao et al. reported the crystal structure of the
ribosome trapped with EF-G in the posttranslocational state,
which provides new details about the role of several
ribosomal elements in EF-G function. Recent Nobel prize
winner Venki Ramakrishnan, the senior author of both
studies, discussed the work in a related
podcast interview.
That
Fizzy Sensation
Carbonated beverages are immensely
popular not only because of the various flavors they impart,
but also because of the fizzy sensation they produce in our
mouths. In a Report in the 16 Oct 2009 Science,
Chandrashekar et al. offered new insights into the
molecular mechanisms that contribute to this distinctive
taste sensation. The team genetically ablated specific sets
of taste-receptor cells in mice and found that mice that
lacked sour-sensing taste cells showed no neural response to
carbonation, indicating that without these cells they
couldn't detect carbon dioxide. A screen for genes
specifically expressed in the sour-sensing cells revealed
the gene encoding carbonic anhydrase 4, an enzyme that
converts carbon dioxide into bicarbonate ions and free
protons. Knockout animals not expressing this gene also
showed diminished carbon dioxide sensation. The protons
produced by the enzyme appear to be the actual molecules
sensed by the sour-sensitive cells. This process, combined
with the response of mechanoreceptive cells to bursting
carbon dioxide bubbles in the mouth, seem to be the source
of the popular fizzy sensation. A
podcast interview with senior author Charles Zuker and a
New story by G. Miller highlighted the findings.
Persistent Currents
In superconductors, currents are expected to flow
persistently without dissipation. Quantum mechanics predicts
that such persistent currents should also exist in normal,
nonsuperconducting metal rings. However, the predicted
effect is small, which has made the detection of these
currents difficult. In a Report in the 9 Oct 2009 Science,
Bleszynski-Jayich et al. reported on the development of
a new technique for detecting persistent currents that
employs nanoscale cantilevers to indirectly measure the
current through changes in the magnetic force it produces as
it flows through the metal ring (listen to the
podcast interview with senior author Jack Harris). The
team fabricated either a single ring or an array of aluminum
rings near the tip of a nanomechanical cantilever that
serves as an oscillator. When a magnetic field is applied,
the interaction of the persistent current with the field
leads to a very small torque on the cantilever, which in
turn changes its oscillation frequency slightly, but
measurably. Using this technique, the team achieved a
sensitivity about 100 times greater than previous scanning
microscope-based measurements. Moreover, they were able to
measure the currents over a wide range of temperatures, ring
sizes, and magnetic fields. An accompanying
Perspective by N. O. Birge highlighted the study.
Lunar Water
Since the first sample return missions of the 1960s, lunar
scientists have operated under the presumption that the Moon
is entirely dry. Now, three Reports in the 23 Oct 2009
Science (published online 24 Sep) challenge that
notion, presenting infrared spectroscopic measurements from
three spacecraft that provide strong evidence for water on
the lunar surface (see the 24 Sep
ScienceNOW story by R. Kerr).
Clark examined data obtained by the Visual and Infrared
Mapping Spectrometer on Cassini during its flyby of the Moon
in 1999;
Pieters et al. and
Sunshine et al. analyzed infrared mapping data captured
by the Moon Mineralogy Mapper onboard India’s Chandrayaan-1
spacecraft and a high-resolution spectrometer onboard Deep
Impact, respectively. All three teams detected absorption
spectra indicative of water or hydroxyl (or both) across
much of the Moon's surface. Some variability in water
abundance is seen over the course of the lunar day. In
addition, all three studies show that the water-related
absorption increases toward the cold lunar poles. Taken
together, the data imply that solar wind is depositing
and/or forming water and OH -- through hydrogen interacting
with surface rock -- and that this water is in a process of
dynamic migration across the lunar surface. An accompanying
Perspective by P.G. Lucey highlighted the findings.
Inheritance and Inequality
Contemporary societies exhibit varying degrees of economic
inequality, but the sources of these disparities are poorly
understood. In a Report in the 30 Oct 2009 Science,
Borgerhoff Mulder et al. presented a descriptive model
of how the transmission of wealth from one generation to
another affects economic inequality in small-scale
societies. The researchers considered three different types
of wealth -- relational wealth (based on social networks),
material wealth (based on land and livestock), and embodied
wealth (based on physical and cognitive capacity) -- in four
types of small-scale societies in which livelihoods depended
primarily on hunting, herding, farming, or horticulture.
They found that the four types of societies display
distinctive patterns of wealth transmission and that these
patterns are associated with different extents of
inequality. More specifically, their analysis revealed that
the inheritance of wealth and wealth inequality are
substantial among pastoral and agrarian societies, in which
institutions enable the intergenerational transmission of
material assets such as land and livestock, but are limited
among horticultural and foraging peoples. Thus, differences
in the ways that people derive their livelihood, as well as
differences in the norms and institutions making up the
economic systems both contribute to this pattern. A
podcast interview with lead author Monique Borgerhoff
Mulder and a
Perspective by D. Acemoglu and J. Robinson discussed the
findings in further detail.
Advances
in Neuroscience Methods
Special Issue Introduction
Ever since human beings began to systematically investigate
the natural world, the introduction of new techniques has
pushed the boundaries of our knowledge and revolutionized
the way in which we see ourselves and our place in the
cosmos. In a special section of the 16 Oct 2000 Science,
several leading neuroscientists, each of whom investigates a
different organizational level of the nervous system,
discussed the latest methodological developments in their
area of interest. Their Review articles delved into new
molecular and cellular methods that allow the tagging and
direct study of neurons in vivo, the promise of optogenetics
for providing novel insights into the organization of neural
circuits and the regulation of their interactions, and
achievements in functional neuroimaging. A related
Perspective looked at the latest advances in computational
neuroscience and how improvements in our approaches to
neuronal modeling can help us make more accurate and
testable predictions about the behavior of neurons. Finally,
two News features highlighted efforts to develop biomarkers
for Alzheimer's disease and high-throughput tools to
visualize neural activity in the brains of fruit flies.
Viral Link to Chronic Fatigue
As its name implies, chronic fatigue syndrome (CFS) is
characterized by debilitating fatigue that can persist for
many years. Although often linked to immune system
dysfunction, the cause(s) of the disorder remain mysterious.
In a Report in the 23 Oct 2009
Science (published online 8 Oct),
Lombardi et al. presented a potential new lead. The team
found that blood samples from 68 of 101 CFS patients
contained DNA from a human retrovirus called xenotropic
murine leukemia virus–related virus (XMRV), which is
genetically similar to a murine leukemia virus. Cell culture
assays confirmed that XMRV derived from CFS patient plasma
and from T and B lymphocytes was infectious. Interestingly,
nearly 4% of the 218 healthy donors tested were also
positive for XMRV, which suggests that the virus may be
present in a significant proportion of the general
population. As noted in an accompanying
Perspective by J. M. Coffin and J. P. Stoye, CFS is not
the first human disease to which XMRV has been linked: it
has also been linked to an aggressive form of prostate
cancer (see the related
News story by S. Kean in the 9 Oct issue). Both
laboratory and epidemiological studies are needed to
determine whether the virus plays a causative role in these
diseases.
Blood
Vessel Split
In vertebrates, blood vessels form a tree-like, tubular
network consisting of arteries, capillaries, and veins. This
vasculature has been thought to involve just two mechanisms
of blood vessel formation: de novo formation from
endothelial precursor cells (vaculogenesis), or growth and
remodeling of existing vessels (angiogenesis). Now, in a
Report in the 9 Oct 2009 Science,
Herbert et al. report on a third mechanism in zebrafish
in which two distinct, unconnected blood vessels can be
derived from a single precursor vessel. Using
high-resolution imaging to examine zebrafish vascular
development, the team found that early zebrafish embryos
initially contain only a single blood vessel in the location
of the aorta. The first embryonic vein forms by selective
sprouting of progenitor cells from this precursor vessel,
followed by vessel segregation. Several vascular endothelial
growth factors and signaling pathways, including ephrin and
Notch signaling, coordinate the sorting and segregation of a
mixture of arterial-fated and venous-fated precursor cells
into the distinct arterial and venous vessels. These
findings provide a mechanistic framework for how mixed
populations of cells can coordinate their behavior to
segregate and form distinct blood vessels. An accompanying
Perspective by R. Benedito and R. H. Adams highlighted
the Report.
Impacts of Emissions
Evaluating how to change or control climate-altering
atmospheric emissions requires quantitative knowledge of the
diverse direct and indirect effects of these pollutants.
Most past calculations of this type have considered the
radiative forcing of a specific emission and its atmospheric
lifetime, but a Report by
Shindell et al. in the 30 Oct 2009 Science now
shows that interactions between atmospheric gases and
aerosols substantially alter the relative importance of
various emissions. Using sophisticated atmospheric
composition and climate modeling, the team found that
methane emissions in particular have a larger impact than
that used in current carbon-trading schemes or in the Kyoto
Protocol. These findings suggest that gas-aerosol
interactions should be considered in assessments of multigas
mitigation policies as well as any separate efforts to
mitigate warming from short-lived pollutants. The
researchers noted that additional processes such as mixing
between aerosol types, formation of secondary organic
aerosols, and interactions between pollutants and ecosystems
should also be considered as they become better understood.
Two related Perspectives further discussed the complex
relationships between air pollution and climate change.
Arneth et al. considered whether air pollution control
measures will accelerate or mitigate climate change, while
Parrish and Zhu discussed how efforts to lessen the
severe health impacts of air pollution can also provide
opportunities for climate change mitigation.
In Science Signaling
In its 20 Oct 2009 issue, Science Signaling turned
the spotlight on neuroscience, featuring articles about how
traditional genetics techniques can be used to identify
unexpected connections between signaling processes and
diseases affecting the brain. In experiments with knockout
mice,
Berna-Erro et al. demonstrated that the calcium sensor
STIM2 plays a critical role in neuronal calcium handling,
but may also contribute to neurological damage associated
with loss of blood flow, such as that which occurs during
stroke. A
Review by Thathiah and De Strooper highlighted how mouse
models of Alzheimer's disease have revealed insight into the
molecular mechanism of beta-amyloid toxicity and the
cognitive defects associated with this neurodegenerative
disease. And a
Perspective by Ryder and Faundez noted that genetic
polymorphisms in some genes linked to schizophrenia
implicate dysfunctional endosomal trafficking as a
contributing factor to this devastating disease. An
Editorial Guide by N.R. Gough considered how these and
other Science Signaling articles contribute to our
understanding of signaling processes associated with
neurological diseases.
Also in Science Signaling
this month:
--
Lee et al. showed that a protein kinase helps rice
seedlings survive flooding (6 Oct 2009)
--
Meyn and Smithgall reported that distinct Src kinases
regulate the differentiation of embryonic stem cells (13 Oct
2009)
--
N.R. Leslie discussed recent work implicating a guanine
exchange factor in the inhibition of the tumor suppressor
PTEN (27 Oct 2009)
Image credits (in order of appearance): Science
cover, 2 October 2009 (Image: © T. White, 2008);
Science/AAAS; iStockphoto.com; Science cover
23 October 2009 (Image: NASA/ISRO/Brown University/R. N.
Clark, USGS); K. Sutliff/Science; Y. Greenberg/Science
