Humans have influenced Earth's history for thousands of years, though some scientists count changes of the last two centuries as especially notable. (Left to right) Universal History Archive/UIG via Getty Images; Hulton Archive/Getty Images; Liszt Collection/Heritage Images/Getty Images; Joint Task Force One/AP hide caption

itoggle caption (Left to right) Universal History Archive/UIG via Getty Images; Hulton Archive/Getty Images; Liszt Collection/Heritage Images/Getty Images; Joint Task Force One/AP

NASA astronaut Scott Kelly is seen inside a Soyuz simulator at the Gagarin Cosmonaut Training Center on March 4 in Star City, Russia. Kelly, along with Russian cosmonaut Mikhail Kornienko of the Russian Federal Space Agency, are scheduled for launch Friday aboard a Soyuz TMA-16M spacecraft from the Baikonur Cosmodrome in Kazakhstan. NASA/Bill Ingalls hide caption

itoggle caption NASA/Bill Ingalls

Mars, anyone? Six researchers from the Mars Society sport their best space duds during this 2014 simulation of the conditions that explorers of the Red Planet might face. (From left) Ian Silversides, Anastasiya Stepanova, Alexandre Mangeot and Claude-Michel Laroche. Micke Sebastien/Paris Match via Getty Images hide caption

itoggle caption Micke Sebastien/Paris Match via Getty Images

The size of the brain of a chimpanzee (right) is considerably smaller than that of a human brain. Probably multiple stretches of DNA help determine that, geneticists say. Science Photo Library/Corbis hide caption

itoggle caption Science Photo Library/Corbis

The eruption of Mount Pinatubo in the Philippines in 1991 spewed almost 20 million tons of sulfur dioxide into the atmosphere, causing worldwide temperatures to drop half a degree on average. Arlan Naeg/AFP/Getty Images hide caption

itoggle caption Arlan Naeg/AFP/Getty Images

Stars over the Cerro Tololo Inter-American Observatory in Chile. Sheppard and Trujillo used the new Dark Energy Camera (DECam) on a telescope there to find the distant dwarf planet 2012 VP 113. Reidar Hahn/Fermilab hide caption

itoggle caption Reidar Hahn/Fermilab

Ultrasound is often used for prenatal screening. It's just one of several prenatal screenings available to pregnant women. iStockphoto hide caption

itoggle caption iStockphoto

An example of a human precision grip — grasping a first metacarpal from the thumb of a specimen of Australopithecus africanus that's thought to be 2 to 3 million years old. T.L. Kivell & M. Skinner hide caption

itoggle caption T.L. Kivell & M. Skinner

Bar-headed geese tend to follow the sharp ups and downs of the Himalayas as they migrate, research finds. John Downer/Nature Picture Library/Corbis hide caption

itoggle caption John Downer/Nature Picture Library/Corbis

The newly described L. larvaepartus (male, left, and female) from Indonesia's island of Sulawesi. Odd, sure, but at least they don't use their stomachs as breeding chambers, as some other frogs do. Jim McGuire/UC Berkeley hide caption

itoggle caption Jim McGuire/UC Berkeley