American Scientist

In this roundup, managing editor Stacey Lutkoski summarizes notable recent developments in scientific research, selected from reports compiled in the free electronic newsletter Sigma Xi SmartBrief.

Discovering New Earths

The Transiting Exoplanet Survey Satellite (TESS), launched April 18, is already discovering possible Earth-like exoplanets. TESS’s mission is to search for planets as small as Earth orbiting the nearest and brightest stars outside our Solar System (see What’s Next for Finding Other Earth-like Worlds). On September 18, the NASA TESS team announced their first candidate planet: a super-Earth orbiting the yellow dwarf star π Mensae, about 60 lightyears away from Earth. Two days later, they announced a second candidate, this time orbiting the red dwarf star LHS 3844, about 49 lightyears from Earth. The second planet, nicknamed “hot Earth,” is about the same size as Earth and is tightly orbiting its star. The speed with which these two discoveries were made bodes well for future exoplanet detection during TESS’s two-year mission.

Huang, C. X., et al. 2018. TESS discovery of a transiting super-Earth in the π Mensae system. https://arxiv.org/abs/1809.05967 (September 18)
Vanderspek, R., et al. 2018. TESS discovery of an ultra-short-period planet around the nearby M dwarf LHS 3844. https://arxiv.org/abs/1809.07242 (September 20)

Denisovan-Neanderthal Mixing

Scientists have identified evidence of the first known Denisovan-Neanderthal offspring. Genetic analysis of a 50,000-year-old bone fragment found in the Denisova Cave, Siberia, revealed that it came from a girl who had a Denisovan father and a Neanderthal mother. Based on the cortical thickness of the bone fragment, known as Denisova 11, researchers infer that the girl was at least 13 years old when she died. Previous studies had found traces of Neanderthal DNA in another Denisovan bone fragment (Denisova 3), indicating that the two groups had mixed at least once before; however, this discovery of first-generation offspring suggests that mixing between the hominin groups was ongoing. Further analysis of Denisova 11 indicates that her father had a Neanderthal ancestor as well, possibly 300–600 generations before his lifetime. Neanderthals primarily inhabited western Eurasia, whereas Denisovans inhabited eastern Eurasia, so their interaction was likely limited; however, the DNA evidence suggests that mixing was common among Late Pleistocene hominin groups when they encountered one another.

Slon, V., et al. 2018. The genome of the offspring of a Neanderthal mother and a Denisovan father. Nature 561:113–116 (August 22)

“Turning Off” Languages

Bilingual individuals switch between languages by “turning off” one language and “turning on” another. A study of bilingual individuals found that it takes more effort to disengage from a language than to engage in a new one. The study focused on people who are fluent in American Sign Language and English because they are able to communicate in both languages simultaneously. Using neuroimaging, researchers found that turning a language “off” called for increased activity in the anterior cingulate cortex and the dorsolateral prefrontal cortex regions of the brain, but switching a language “on” did not require increased activity in those areas. The results suggest that it is more difficult to turn a language “off” than to turn one “on,” and that it is therefore not necessarily more difficult for someone to communicate in two languages simultaneously than it is to speak in only one.

Blanco-Elorrieta, E., K. Emmorey, and L. Pylkkänen. 2018. Language switching decomposed through MEG and evidence from bimodal bilinguals. Proceedings of the National Academy of Sciences of the U.S.A. doi:10.1073/pnas.1809779115

Mapping Global Deforestation

Much of global forest loss is the result of a permanent shift in land use to commodity production. Researchers used images from Google Earth to examine changes in forest coverage between the years 2001 and 2015. They developed a computer program that analyzed the satellite images and tracked global forest fluctuations as well as the causes behind the changes.

They found that commodity-driven deforestation constituted 27 percent of forest disturbance. Agriculture, particularly industrial palm oil and soybean farming, was responsible for the majority of the permanent forest loss. This type of deforestation was prevalent in South America and Southeast Asia. In contrast, subsistence farming in sub- Saharan Africa did not lead to permanent tree loss, because the small farms would move to new locations over time, allowing trees to return. The other 73 percent of tree loss was determined not to be permanent. For example, timber production in Europe and North America includes seedlings planted to replace what is harvested, and forests lost to wildfires lead to new growth (see page 344). The data from the forest-loss maps can be used to verify whether companies are upholding their commitments to sustainability, and to track carbon cycling, an important factor in climate change.

Curtis, P. G., C. M. Slay, N. L. Harris, A. Tyukavina, and M. C. Hansen. Clarifying drivers of global forest loss. Science 361(6407):1108–1111 (September 14)