Top scientific discoveries of the decade

New Human Relatives

The human family tree expanded significantly in the past decade, with fossils of new hominin species discovered in Africa and the Philippines. The decade began with the discovery and identification of Australopithecus sediba, a hominin species that lived nearly two million years ago in present-day South Africa. Matthew Berger, the son of paleoanthropologist Lee Berger, stumbled upon the first fossil of the species, a right clavicle, in 2008, when he was only 9 years old. A team then unearthed more fossils from the individual, a young boy, including a well-preserved skull, and A. sediba was described by Lee Berger and colleagues in 2010. The species represents a transitionary phase between the genus Australopithecus and the genus Homo, with some traits of the older primate group but a style of walking that resembled modern humans.

Also discovered in South Africa by a team led by Berger, Homo naledi lived much more recently, some 335,000 to 236,000 years ago, meaning it may have overlapped with our own species, Homo sapiens. The species, first discovered in the Rising Star Cave system in 2013 and described in 2015, also had a mix of primitive and modern features, such as a small brain case (about one-third the size of Homo sapiens) and a large body for the time, weighing approximately 100 pounds and standing up to five feet tall. The smaller Homo luzonensis (three to four feet tall) lived in the Philippines some 50,000 to 67,000 years ago, overlapping with several species of hominin. The first H. luzonensis fossils were originally identified as Homo sapiens, but a 2019 analysis determined that the bones belonged to an entirely unknown species.

These three major finds in the last ten years suggest that the bones of more species of ancient human relatives are likely hidden in the caves and sediment deposits of the world, waiting to be discovered.

Taking Measure of the Cosmos

When Albert Einstein first published the general theory of relativity in 1915, he likely couldn’t have imagined that 100 years later, astronomers would test the theory’s predictions with some of the most sophisticated instruments ever built—and the theory would pass each test. General relativity describes the universe as a “fabric” of space-time that is warped by large masses. It’s this warping that causes gravity, rather than an internal property of mass as Isaac Newton thought.

One prediction of this model is that the acceleration of masses can cause “ripples” in space-time, or the propagation of gravitational waves. With a large enough mass, such as a black hole or a neutron star, these ripples may even be detected by astronomers on Earth. In September 2015, the LIGO and Virgo collaboration detected gravitational waves for the first time, propagating from a pair of merging black holes some 1.3 billion light-years away. Since then, the two instruments have detected several additional gravitational waves, including one from a two merging neutron stars.

Another prediction of general relativity—one that Einstein himself famously doubted—is the existence of black holes at all, or points of gravitational collapse in space with infinite density and infinitesimal volume. These objects consume all matter and light that strays too close, creating a disk of superheated material falling into the black hole. In 2017, the Event Horizon Telescope collaboration—a network of linked radio telescopes around the world—took observations that would later result in the first image of the environment around a black hole, released in April 2019.

Editing Genes

Ever since the double-helix structure of DNA was revealed in the early 1950s, scientists have hypothesized about the possibility of artificially modifying DNA to change the functions of an organism. The first approved gene therapy trial occurred in 1990, when a four-year-old girl had her own white blood cells removed, augmented with the genes that produce an enzyme called adenosine deaminase (ADA), and then reinjected into her body to treat ADA deficiency, a genetic condition that hampers the immune system’s ability to fight disease. The patient’s body began producing the ADA enzyme, but new white blood cells with the corrected gene were not produced, and she had to continue receiving injections.

Now, genetic engineering is more precise and available than ever before, thanks in large part to a new tool first used to modify eukaryotic cells (complex cells with a nucleus) in 2013: CRISPR-Cas9. The gene editing tool works by locating a targeted section of DNA and “cutting” out that section with the Cas9 enzyme. An optional third step involves replacing the deleted section of DNA with new genetic material. The technique can be used for a wide range of applications, from increasing the muscle mass of livestock, to producing resistant and fruitful crops, to treating diseases like cancer by removing a patient’s immune system cells, modifying them to better fight a disease, and reinjecting them into the patient’s body.

In late 2018, Chinese researchers led by He Jiankui announced that they had used CRISPR-Cas9 to genetically modify human embryos, which were then transferred to a woman’s uterus and resulted in the birth of twin girls—the first gene-edited babies. The twins’ genomes were modified to make the girls more resistant to HIV, although the genetic alterations may have also resulted in unintended changes. The work was widely condemned by the scientific community as unethical and dangerous, revealing a need for stricter regulations for how these powerful new tools are used, particularly when it comes to changing the DNA of embryos and using those embryos to birth live children.

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