American Scientist

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

Tiny Fossil Footprints

Amateur paleontologist Gloria Melanson was walking along the Joggins Fossil Cliffs in Nova Scotia, Canada—well known for the richness of Carboniferous Period terrestrial vertebrate fossils that are found there—when she noticed 30 tiny fossilized footprints in a slab of siltstone. They have been attributed to a juvenile amphibian of the species Batrachichnus salamandroides. The prints are 1 to 3 millimeters wide, making them the smallest tetrapod fossil footprints ever found. The entire body length of the animal that left the tracks was estimated to be 8 millimeters.

Stimson, M., et al. The smallest known tetrapod footprints: Batrachichnus salamandroides from the Carboniferous of Joggins, Nova Scotia, Canada. Ichnos 19:127–140 (August 29)

What Genomes Do

Once the Human Genome Project was complete, researchers had identified the 3 billion nucleotides of the human genome, but their functions were still unknown. Although 20,000 genes that code for proteins were identified, those only accounted for roughly 1 percent of the genome. A research group called ENCODE was started in 2003 to reveal the functions of the rest of the genome, and they recently announced that they have successfully identified the function of about 80 percent of the genome. Molecular biologists once thought that noncoding DNA served no purpose, but over the past decade, this paradigm has shifted to recognizing an incredible complexity in gene regulation. DNA sequences identified include 400,000 enhancer sites that regulate distant genes and 70,000 promoter sites that immediately precede the gene they regulate. Researchers ran experiments to confirmfunction in at least 147 cell types, because how a DNA sequence is used in one cell type may not be consistent with its use in another cell. Understanding the complexity of our genetic blueprint is fascinating, overwhelming and instrumental to our understanding of human disease.

The ENCODE Project Consortium. An integrated encyclopedia of DNA elements in the human genome. Nature 489:57–74 (September 5)

Planets with Two Suns

Most stars come in pairs that orbit each other, unlike our solitary Sun. Although a single planet orbiting a pair of stars, called a circumbinary planet, had been found before, no one had found a system of more than one planet orbiting a pair of stars. Using data from the Kepler space telescope, astronomers have identified the first circumbinary multiplanet system, which they named Kepler-47. This multiplanet, multistar system is 5,000 light years away in the constellation Cygnus. Of the two planets, the one further out from the stars has a large orbit of 303.2 days, making it the first circumbinary one found in the “habitable zone,” the range of distances from a star that a planet could harbor liquid water. Although this planet is likely gaseous and not suitable to life, its location demonstrates that there could be liquid water on planets in binary star systems.

Orosz, J. A., et al. Kepler-47: A transiting circumbinary multiplanet system. Science (Published online August 28)

Ancient Human DNA

The genome of an archaic human female from a group called the Denisovans, who lived about 41,000 years ago, has been sequenced with a new method of DNA library preparation, using genetic material from a finger bone. Because ancient DNA is highly fragmented, the preparation method was novel in its prevention of lost genetic material and its low error rate given such a degraded sample. This increased accuracy permitted a confident estimation of Denisovan genetic diversity that was impossible with earlier attempts at sequencing this ancient DNA.

The genome from Denisovans, whose remains were found in Denisova Cave in Siberia, was compared to those of Neandertals and modern humans from around the world. The study revealed that Denisovans are most closely related to Neandertals and confirmed evidence of mating between archaic and modern humans. Although all present-day humans worldwide inherited DNA from Neandertals, only modern humans in Southeast Asia and Oceania inherited Denisovan DNA. Very few genome changes in modern humans differentiate them from ancient humans. These differences may explain the rapid spread of modern humans. Eight of these changes are in DNA related to brain function. The advent of this method for genome sequencing with very trace amounts of degraded DNA will allow huge advances in understanding evolution, both human and otherwise.

Meyer, M., et al. A high-coverage genome sequence from an archaic Denisovan individual. Science (Published online August 31)

Killer Whale Menopause

Female killer whales live long after their reproductive years are over. A recent study showed that mother whales’ adult sons live longer and produce more offspring when the mother has a long post-menopausal phase. This trend was not observed for daughters of the same whales. The mechanisms by which mothers increase their adult sons’ survival are unclear, but hypothesized reasons include food foraging assistance and support during dangerous situations.

Foster, E. A., et al. Adaptive and prolonged postreproductive life span in killer whales. Science 337:1313 (September 14)