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Geologic Time

  • 205

    Birds First Take Wing

    Birds First Take Wing
    Birds became airborne some 150 million years ago, the earliest record coming from a well-preserved fossil discovered in Germany in 1861. The fossil showed a creature with unmistakable wing feathers but also reptilian features, such as a bony tail, arm claws, and sharp teeth. Named Archaeopteryx ("ancient wing"), scientists think the animal was capable of full flight.
  • 248

    Dinosaurs Take First Steps

    Dinosaurs Take First Steps
    The age of the dinosaurs dawned some 240 million years ago—the time to which the oldest known dinosaur dates. Identified from fossil fragments in Madagascar, the kangaroo-size animal belonged to a group of primitive plant-eaters known as prosauropods. Better-known pioneering dinosaurs come from fossils uncovered in Argentina.
  • 360

    Single Supercontinent Comes Together

    Single Supercontinent Comes Together
    During the Carboniferous, the Earth's major landmasses started coming together to create a single, vast supercontinent known as Pangaea. At the beginning of the period, most of the continents were already lumped together into Gondwana, the bulk of which lay over the South Pole. Euramerica, which included North America and northern Europe, gradually edged closer to this much larger, much cooler landmass.
  • 500

    Earliest Cartilaginous Fish Evolve

    Earliest Cartilaginous Fish Evolve
    The ancestors of sharks first swam about 450 million years ago. Known as cartilaginous fish because their skeletons were made of cartilage, not bone, sharks and rays left very few early fossils. The oldest well-preserved specimen dates to 409 million years ago. Discovered in New Brunswick, Canada, Doliodus had spines on its pectoral fins and may have resembled a modern angel shark
  • 570

    Hard-Shelled Mollusks Appear

    Hard-Shelled Mollusks Appear
    Shelled mollusks show up in the fossil record 545 million years ago. The origins and earliest evolution of this diverse group, which includes clams, snails, squid, and octopuses, remain unclear. Among the first fossil specimens were the monoplacophorans—the ancestors of many shelled mollusks alive today. Thought to be descended from annelids, monoplacophorans lacked eyes and moved on a rounded foot under a simple, limpet-like shell.
  • Modern Humans Are Born

    Modern Humans Are Born
    Fossils indicate our own species, Homo sapiens, arose in eastern Africa some 190,000 years ago. First venturing beyond Africa about 70,000 years ago, early modern humans eventually penetrated as far as Australia and South America. Hunter-gatherer groups in Europe and western Asia came into contact with closely related Neandertals, who subsequently died out—possibly because the newcomers outthought and outcompeted them.
  • Dinosaurs Go Extinct

    Dinosaurs Go Extinct
    The end of the Cretaceous, 65 million years ago, was signaled by a massive extinction event that wiped out the non-avian dinosaurs. Up to 50 percent of the planet's animal and plant species disappeared. Other notable victims included giant marine reptiles, mosasaurs, and the flying pterosaurs. What caused this huge loss of life isn't certain. Climate change is strongly implicated, with global cooling linked to declines in the abundance of some species leading up to the extinction event.
  • Oxygen Levels Rise

    Oxygen Levels Rise
    The Earth was a suffocating place until oxygen in the atmosphere slowly began to climb from almost nonexistent levels about 2.5 billion years ago. Breathable air is thought to have been created by cyanobacteria, single-celled microbes living in the sea.These bacteria harnessed the energy of the sun through photosynthesis—the biochemical process used by plants—producing oxygen as a by-product.
  • World's Oldest Rock Forms

    World's Oldest Rock Forms
    The world's oldest known rock structure represents early chunks of the Earth's crust made when molten lava rose from cracks in the seafloor. Discovered in western Greenland, these formations have been dated to 3.8 billion years ago. Rocks in the same area are also thought to record the first fossil traces of bacterial life.
  • Eath Forms

    Eath Forms
    Earth was created between 4.5 and 4.6 billion years ago, when our solar system took shape around the sun. Born from exploding stars called supernovae, the new star gathered a swirling disk of dust and gases around it. As the sun heated up, this orbiting matter accumulated, and successively larger bodies collided to eventually create protoplanets with their own gravitational pull.