Highlights in PNAS

New for March 13, 2019

Anthropology

Old World monkeys and dental evolution

Knowledge about the origin and evolution of Old World monkeys, a member of the family Cercopithecoidea, is limited because of a meager fossil record that until now consisted of only two fossilized teeth from the first approximately 12 million years of the group’s existence. D. Tab Rasmussen et al. found 22-million-year-old cercopithecoid dental remains that help fill in this long-standing gap in the fossil record. The authors examined the fossil monkey’s dentition via phylogenetic analysis, CT scans, and 3D reconstructions. The teeth, which were found at three different sites in Nakwai, Kenya, lacked a trait shared among all modern Old World monkeys, namely bilophodonty, or the presence of two transverse crests that connect the four cusps of a molar. The authors named the species Alophia metios, given that the monkeys did not have lower molar cross lophids. Because Alophia lacked lophs, they probably relied on cusps to process food. The findings suggest that Alophia may have fed on hard objects, such as fruits and seeds. Monkeys may have evolved bilophodonty when they began incorporating soft leaves in their diet, according to the authors. — M.S.

"Primitive Old World monkey from the earliest Miocene of Kenya and the evolution of cercopithecoid bilophodonty," by D. Tab Rasmussen, et al.
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Developmental Biology

Shell shape diversity in gastropods

Gastropods generally have one helically coiled shell, but within that basic geometrical ground plan, there is immense variation in shape, size, and ornamentation. The variations have been mathematically and computationally modeled, but the developmental mechanisms that control shell morphology are unknown. Adam Johnson et al. examined cell division patterns in the shell-secreting tissue of two gastropods with different shell shapes. In the basket whelk Tritia, which has a helically coiled shell, the shell margin contains two regions, the aperture growth zone (AGZ) and the mantle epithelium growth zone (MEGZ), that are characterized by high levels of cellular proliferation. Cell divisions that are biased parallel to shell growth occur to a greater extent on the shell margin’s left side in the AGZ, whereas cell division angles are biased to the right in the MEGZ. However, the left–right asymmetries in the growth-zone patterns are different in Planorbella sp., which has a distinct coiling geometry compared with Tritia. The results suggest that asymmetric growth patterns may contribute to shell shape in gastropods, potentially representing cellular mechanisms underlying the evolution of diverse shell morphologies. — J.W.

"Growth and morphogenesis of the gastropod shell," by Adam B. Johnson, Nina S. Fogel, and J. David Lambert
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Environmental Sciences; Sustainability Science

Timing of precipitation pattern change

Climate change is expected to affect precipitation patterns in regions throughout the world, albeit with differing effects in different regions. Such precipitation changes are of concern in agricultural areas, where precipitation declines would lead to declines in crop productivity. Maisa Rojas et al. used ensembles of multiple climate models to identify regions subject to precipitation changes and estimate the time when such climate change effects may emerge from natural variation. The authors also correlated the results to agricultural growing seasons of four major crops to assess the impacts to grain production. The models suggested that patterns of increased precipitation in high latitudes, including areas in North America and Europe, could emerge as early as the 2020s, or have already emerged. Patterns of decreased precipitation in areas such as the Mediterranean, western Mexico, Chile, South Africa, and Australia could emerge by midcentury. Precipitation pattern changes emerged even in the lowest carbon emissions scenario. According to the authors, identifying precipitation pattern emergence on both geographical and temporal scales can aid timely regional adaptation strategies to anticipate future droughts or floods and secure global food supplies. — P.G.

"Emergence of robust precipitation changes across crop production areas in the 21st century," by Maisa Rojas, Fabrice Lambert, Julian Ramirez-Villegas, and Andrew J. Challinor
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Sustainability Science

Racial–ethnic inequity in air pollution

Exposure to fine particulate matter (PM_2.5), the leading environmental health risk factor in the United States, is inequitably distributed among racial–ethnic groups. Consumption of goods and services, which contribute to PM_2.5 levels, is also unequal across racial–ethnic groups, but the relationship between inequality in consumption and inequality in exposure has not been explored previously. Christopher Tessum et al. attributed all PM_2.5-related emissions in the United States to specific end-use activities and end-user groups. The authors calculated the “pollution inequity” of racial–ethnic groups as the fractional difference between a group’s exposure to PM_2.5 and its contribution to total PM_2.5 exposure. On average, blacks and Hispanics experienced 56% and 63% greater PM_2.5 exposure than their contributions, respectively, while non-Hispanic whites experienced 17% less exposure than their contribution. Pollution inequity depended at least as much on differences in consumption among groups as on differences in exposure. Differences in the types of goods and services consumed by each group were less important than differences in their overall amount of consumption in determining groups’ contributions to PM_2.5 exposure. Even though pollution exposure for all three groups declined approximately 50% during 2003–2015, pollution inequity remains high, according to the authors. — B.D.

"Inequity in consumption of goods and services adds to racial–ethnic disparities in air pollution exposure," by Christopher W. Tessum, et al.
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Sustainability Science

Risk perceptions of extreme heat

Although extreme heat is the leading cause of weather-related deaths in the United States, many individuals fail to perceive heat-related risks. Peter Howe, Jennifer Marlon, et al. analyzed survey data from 9,217 respondents across the United States to determine what factors affect risk perception of extreme heat and how perceptions vary across the country. Individuals living in warm climates had higher risk perceptions than individuals living in cool climates, even though individuals in colder regions tend to be at higher risk during heat waves. Using high-resolution maps, the authors also found that risk perceptions can vary as much between neighborhoods in the same city as between states with different climates. For example, individuals living in high-income neighborhoods had lower risk perceptions than individuals living in less affluent areas. Compared with neighborhoods with younger residents, neighborhoods with older residents tended to have lower risk perceptions, even though older people are more vulnerable to heat. The findings suggest that understanding the geographic diversity of perceived risks of extreme heat could lead to ways to promote adaptive behaviors during heat waves, according to the authors. — M.S.

"Public perceptions of the health risks of extreme heat across US states, counties, and neighborhoods," by Peter D. Howe, Jennifer R. Marlon, Xinran Wang, and Anthony Leiserowitz
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Some of the highlights have previously appeared on the PNAS media tipsheet. The articles in PNAS report original research by independent authors and do not necessarily represent the view of the National Academy of Sciences.

[03/19]