Charles Robert Darwin (February 12, 1809 – April 19, 1882) is one of the most celebrated and eminent scientists of the past few centuries, with his broadest and most notable influence arising from his theory of evolution by means of natural selection. Darwin’s remarkable investigations and insights obtained during his voyage on the HMS Beagle (1831–1836) led him to theorize about concepts of evolutionary biology and to develop revolutionary ideas related to adaptation and speciation. Although previous scientific thinkers had laid down some of the foundations for Darwin’s work, and others later expanded upon and more fully developed the scientific bases for his conclusions, Darwin set forth and formulated the controversial but coherent ideas about organic evolution that have impacted the world at large. His groundbreaking On the Origin of Species was originally published in 1859. Later, in 1871, Darwin argued in The Descent of Man and Selection in Relation to Sex that humans had evolved just as other organisms had, creating a storm of controversy that continues today. See also: Adaptation (biology); Animal evolution; Organic evolution; Plant evolution; Speciation

Monarch butterflies (Danaus plexippus) exclusively consume milkweed plants (genus Asclepia) both for nutrition and to help deter predators. Milkweed plants contain toxins that accumulate in the butterfly larva, or caterpillars, as the insects feed; those toxins then persist into the adult stage. Monarchs have evolved specific genetic mutations that protect their species from these poisons. Yet various organisms, including black-headed grosbeaks (a kind of bird) and eastern deer mice, as well as parasitic wasp and nematode (worm) species, can withstand the toxin and readily eat monarch butterflies. A new study has now shown how this is possible: all four predators, it turns out, have evolved monarch-like genetic mutations in their respective genomes that confer milkweed toxin resistance. The findings are a remarkable example of convergent evolution across distantly related organisms, whereby organisms independently evolve similar traits as they adapt to comparable environments and selection pressures. See also: Adaptation (biology); Convergent evolution; Evolution; Genetics; Gentianales; Insecta; Lepidoptera; Mutation; Plant-animal interactions; Poisonous plants; Toxin

At the Woranso-Mille paleontological site in northern Ethiopia, researchers have discovered a 3.8 million-year-old cranium (skull) belonging to a primitive hominin known as Australopithecus anamensis. Initially unearthed in 2016, but not reported until dating and other analyses were completed in 2019, the well-preserved fossil is the first skull of this particular species to be found. Paleoanthropologists expect that this discovery will reshape the ancient timeline of human evolutionary history because it reveals the previously unknown facial features of A. anamensis, which now can be compared to the facial morphologies of other ancient hominins. The fossil evidence also opens up the possibility of the coexistence of two early hominins—A. anamensis and A. afarensis—in the same area of Ethiopia. Prior to this cranium discovery, these two species were assumed to have emerged consecutively along a more linear chronology. See also: Anthropology; Australopith; Dating methods; Earliest hominins; Fossil; Fossil humans; Paleontology; Physical anthropology

The emergent discipline of evolutionary epidemiology recognizes that pathogens (disease-causing organisms) are biological entities that can evolve quickly to take advantage of ecological or environmental changes that promote their transmission. Virulence, a measure of a microorganism's ability to infect and sicken a host, also evolves, which allows previously mild diseases to become life-threatening ones, or vice versa. Pathogens and virulence evolve because the life cycle of pathogens is typically very short compared to that of their hosts. The majority of bacteria and viruses replicate numerous times daily, enabling natural selection to favor variants that thrive against the relatively stable, more slowly adapting host. In addition, virulence varies not only from pathogen to pathogen, but also from strain to strain. For example, Escherichia coli can cause fatal foodborne illnesses, yet harmless strains of this bacterium inhabit the intestinal tract of humans. Thus, evolutionary epidemiologists seek to explain the factors contributing to the evolution of pathogens and virulence. See also: Bacteria; Disease; Epidemiology; Escherichia; Escherichia coli outbreaks; Food safety and foodborne illness; Infectious disease; Organic evolution; Pathogen; Virulence; Virus

Recent excavations in Asia unearthed a pristine, three-dimensional fossil of a tiny dinosaur tail with preserved feathers and soft tissues (presumably skin). In 2016, investigators in northern Myanmar (Burma) recovered a piece of Cretaceous-era amber (Fig. 1) containing an approximately 99-million-year-old feathered tail from a theropod dinosaur. (Theropods are carnivorous bipedal saurischian reptiles that first appeared in the Upper Triassic and culminated in the uppermost Cretaceous.) Although the size of the tail measures only 37 mm (1.46 in.), eight vertebrae can be identified, and the morphological structure of the plumage is clearly observable. See also: Amber; Cretaceous; Dinosauria; Fossil; Saurischia

Studies have shown that all classes of vertebrates—and even some invertebrate species—have the cognitive ability to discriminate among different quantities. For example, many kinds of studied animals can distinguish specific numbers of objects in their environment. This mathematical skill is theorized to be important in wide-ranging animal behaviors such as herding, schooling, and flocking; choosing among mates based upon visual characteristics, such as number or size of stripes; and estimating amounts of food sources during foraging. Fewer kinds of animals, however, have been shown to possess the cognitive ability to perform addition and subtraction, which are more complex numerical tasks. To date, researchers have documented this arithmetical ability in primates and birds, as well as in spiders and honeybees. Now a new study has discovered that fish—specifically cichlids and stingrays—can also perform addition and subtraction of a quantity of one in the number space from one to five. The findings are unexpected because neither species is known to have an obvious ecological or behavioral need for this mathematical ability. Furthermore, the findings emphasize that a neocortex—the part of the brain that evolved in mammals and had long been considered necessary for certain higher-level cognitive abilities—is not required for performing basic arithmetical operations. See also: Arithmetic; Batoidea; Brain; Cognition; Evolution; Fish; Mammalia; Mathematics

In the Afar region of Ethiopia, paleoanthropologists unearthed a 2.8 million-year-old jawbone (mandible) that was provisionally assigned to the genus Homo [the genus of human beings, including modern humans (Homo sapiens) and other extinct species]. This fossil discovery, announced in the journal Science in 2015, is extremely important because it fills in a notable gap in the ancient timeline of human evolutionary history. See also: Anthropology; Fossil; Physical anthropology

The extinct members of the mammalian subfamily Glyptodontinae are known as glyptodonts. These prehistoric creatures resembled gigantic, heavily armored armadillos, and they possessed a protective shell of bony plates that measured up to 1.5 meters (4.9 feet) long. Paleontologists estimate that these herbivorous placental mammals grew to be 2 meters (6.6 feet) from head to tail and weighed more than 2000 kilograms (4400 pounds); the armored shell alone weighed 400 kilograms (880 pounds). Many glyptodonts carried a spiked club on the tip of the tail, which they probably used as a defensive weapon. Glyptodonts were found predominantly in South America, with some later forms expanding their range into Central America and North America. Some of the most important glyptodont genera are Glyptodon, the type genus; Doedicurus, the largest fossil specimens; and Glyptotherium, which ventured into North America. See also: Armadillo; Mammalia; Predator-prey interactions; Scale (zoology); South America

The evolutionary transition of certain dinosaurs into modern birds has long been an active area of paleontological research. Because the members of the class Aves are living representatives of theropod dinosaurs, paleontologists have sought fossil evidence to support this evolutionary line of descent. One very critical fossil example is Ichthyornis dispar. This extinct flying bird, which resembled a modern-day seagull, is known exclusively from the latest stages of the Cretaceous (approximately 100–65 million years ago) in North America. The initial discovery of this fossil bird in the 1870s was so notable in providing skeletal evidence of the transitional period between dinosaurs and modern birds that Charles Darwin highlighted the jaw of Ichthyornis as support for the theory of evolution. Now, using the latest fossil finds of this ancient bird in Kansas, along with more precise analyses of fossil specimens previously discovered in the late 1800s, researchers have generated a three-dimensional computer tomography image of this animal's skull, showing that it possessed a beak and other skeletal structures similar to those of modern birds. See also: Animal evolution; Aves; Avian evolution; Computerized tomography; Cretaceous; Cretaceous bird radiation; Dino-Birds; Dinosauria; Evolution of theropod dinosaurs; Extinction; Extinction (paleontology); Ichthyornithiformes; Neornithes; Paleontology

Through study of the fossil record, paleontologists can frequently identify when an ancient species or taxon (a grouping of related organisms) first emerged in evolutionary history. (Paleontologists often prefer the term taxon over species because it is impossible to test whether populations of extinct organisms were reproductively isolated as are living species.) When a taxon disappears from the fossil record, the usual presumption is that it has gone extinct. Occasionally, however, a taxon thought to be extinct is found to be extant again during a later era (it may then become truly extinct during this later period). In extreme cases, organisms believed to be extinct may even show up among today's living biota. In acknowledgment of the New Testament biblical story in which Jesus raised Lazarus from the dead, biologists coined the term "Lazarus effect" to refer to such rediscoveries, and "Lazarus taxon" to describe a species that has been seemingly resurrected from the fossil dead. See also: Animal evolution; Extinction; Extinction (paleontology); Extinction and the fossil record; Fossil; Living fossils; Macroevolution; Paleontology; Systematics