American Scientist

In this roundup, associate editor Katie Burke summarizes notable recent developments in scientific research, selected from reports compiled in the free electronic newsletter Sigma Xi SmartBrief Online: https://www.smartbrief.com/sigmaxi/index.jsp

Americas Linked Earlier

The continents of North and South America are connected by a land bridge in Panama. Most researchers thought it formed about 3 million years ago, but a new study says it formed much earlier—13 to 15 million years ago. Geologists estimated the age of tiny grains of a mineral called zircon from ancient river bedrock in present-day Colombia that they say originated in Panama. They are not sure whether this land bridge was initially transient, and if so, for how long. Once the two Americas were connected, divergent species intermingled, known as the Great American Biological Interchange. For example, South America’s giant ground sloths and armadillos could migrate northward. Animals such as saber-toothed cats migrated southward.

Montes, C., et al. Middle Miocene closure of the Central American Seaway. Science 348:226 (April 10)

Longest Mammalian Migration

A new record has been set for the longest migration among mammals: A female western gray whale traveled round-trip between Mexico and Russia—a total of 13,988 miles in 172 days. The record was previously held by a humpback whale that traveled 10,190 miles. The critically endangered western gray whales feed in the waters off Russia, where biologists tagged seven with satellite monitoring devices. Most of the tags were lost, but the one that remained revealed the epic journey. Because the whales are so rare, little is known about their migration or behavior, and this study provides essential information for conserving them. It also brings into question whether western and eastern gray whales are in fact two separate populations, because their ranges appear to overlap. Eastern gray whales are not considered threatened. The study also highlights new questions about how whales navigate.

Mate, B. R., et al. Critically endangered western gray whales migrate to the eastern North Pacific. Biology Letters doi:10.1098/rsbl.2015.0071 (April 15)

Oldest Fossils Not Actually Fossils

A 3.46-billion-year-old rock from western Australia, once credited as the oldest fossil, turns out not to be a fossil at all. Tube-like structures in the rock were thought to be the cell walls of early cyanobacteria, but new evidence suggests that minerals in hydrothermal systems formed them abiotically. Afterward, carbon attached to the edges, leaving a misleading organic signature that researchers mistook for cell walls, a finding published in Science in 1993. The record for the oldest fossil now goes to a 3.43-billion-year-old rock formation that is also in western Australia and that contains much stronger evidence of life.

Brasier, M. D., J. Antcliffe, M. Saunders, and D. Wacey. Changing the picture of Earth’s earliest fossils (3.5–1.9 Ga) with new approaches and new discoveries. Proceedings of the National Academy of Sciences of the U.S.A. 112:4859 (April 21)

Brine on Mars

After collecting humidity and temperature data for two years, the NASA Curiosity rover has provided compelling evidence for transient liquid brines around the equator of the planet. Salts in the soil lower the freezing temperature of water, which sets up the conditions for this phenomenon. NASA’s Phoenix Mars lander discovered potential brines in 2008, but the presence of subsurface water challenged contemporary climate models. With the new information from the Curiosity rover, the researchers were able to create models consistent with observations: The brines form nocturnally in the upper inches of the Martian soils as the salts absorb water vapor. When temperatures rise in the morning, the water evaporates again. The amount and saltiness of the liquid could not support life.

Martin-Torres, F. J., et al. Transient liquid water and water activity at Gale crater on Mars. Nature Geoscience 8:357 (April 13)

Oldest Stone Tools Found

Upon the discovery of some 149 stone tools in Kenya, the date for archaeological tools is pushed back about 700,000 years, before modern humans emerged. The tools’ discovery is a watershed moment for elucidating behavior and cognition during a previously murky period in human evolution. Not only does the unprecedented archaeological find indicate that these proto-humans had the thinking ability to create sharp-edged tools, but also shows they had a strong grip and good motor control.

Harmand, S., et al. 3.3-million-year-old stone tools from Lomekwi 3, West Turkana, Kenya. Nature 521:310 (May 21)

Method Discerns Twins’ DNA

DNA analysis is an important forensic tool in criminal investigations, but it can be complicated if a suspect has an identical twin. Because the biochemical attachments to DNA can change over time due to environmental factors, such epigenetic differences in identical twins’ genomes can differentiate between them, but previously developed methods were expensive and time-consuming. A new method uses the melting points of the DNA to tell apart twins quickly, cheaply, and simply. Variation in the attachment of methyl groups to DNA affects the expression of genes. Researchers also realized it affects the DNA’s melting point. So, instead of going through the more intensive process of studying methylation differences, the researchers looked for a difference in melting points.

Stewart, L., N. Evans, K. J. Bexon, D. J. van der Meer, and G. A. Williams. Differentiating between monozygotic twins through DNA methylation-specific high-resolution melt curve analysis. Analytical Biochemistry 476:36 (May 1)