The Emergence of Humankind

• The _____ refers to the period in Earth’s history before the existence of humans. • Tracing the history of the world and the various changes and developments that occured during prehistoric times is a daunting task, as evidence is limited primarily to fossils and few artifacts.

• Prehistoric Period

• eon

• explosion of life

• Cambrian Explosion

• first chordates

• The early history of Earth is measured in geologic time. A major division of geologic time is the _____, which encompasses several million years. • The earliest _____ were the Hadean, Archaean, and Protozeroic _____. These three geologic time periods cover the length of time from the Earth's formation 4.5 billion years ago until about 600 million years ago.

• Prehistoric Period

• eon

• explosion of life

• Cambrian Explosion

• first chordates

• The fourth and current geologic eon is the Phanerozoic Eon, which began approximately 541 million years ago (mya) and what experts considere an "_____" on the planet.

• Prehistoric Period

• eon

• explosion of life

• Cambrian Explosion

• first chordates

• The "_____" resulted in the most intense and widespread burst of life on the planet. The Earth at this point already had an oxygen-rich atmosphere. This paved the way for the development of the more complex, multi-cellular life forms. • The period from the first emergence of life forms to the emergence of the dinosaurs, as the dominant life on the planet, was called the Paleozoic Era (541 to 251 mya).

• Prehistoric Period

• eon

• explosion of life

• Cambrian Explosion

• first chordates

• Cambrian (541 to 485 mya) to the Ordovician (485 to 443 mya) Periods, all life on the planet were confined within the oceans. • The earliest life forms consisted of ancient worms and tiny, jawless fishes. These were the _____ - animals with simple spinal cords and were the ancestors of vertebrates or creatures with a backbone.

• Prehistoric Period

• eon

• explosion of life

• Cambrian Explosion

• first chordates

• The _____ are creatures with jointed exoskeletons. The most wellknown of the early _____ were the trilobites-creatures with flat, segmented, plated bodies. • Marine life was diverse and fish became widespread; these early fish resembled modern-day lampreys, as they were jawless and fed by sucking and filtering food from the seabed

• arthropods

• sea scorpion

• Prehistoric sharks

• lobefins

• gigantic creatures

• Many plants and animals made their appearance on land, but they were mostly confined to the seashore. • Huge reefs housed a great number of marine life, among them large arthropods such as the _____, which grew up to two meters long. • Primitive insects, such as centipedes and arachnids also began to appear.

• arthropods

• sea scorpion

• Prehistoric sharks

• lobefins

• gigantic creatures

The fishes underwent their greatest development, and the oceans were filled with a diverse array of marine life. Several species of bony, jawed fish appeared. Among them were ferocious, armored fish like the placoderms, which reached lengths of about 10 meters. _____ also emerged, as well as the bony, scaled fish that became the ancestors of modern day fish.

• arthropods

• sea scorpion

• Prehistoric sharks

• lobefins

• gigantic creatures

A variety of bony fish called _____ left the seas and emerged on the shores; when on land, they breathed through their skins or by means of primitive lungs.

• arthropods

• sea scorpion

• Prehistoric sharks

• lobefins

• gigantic creatures

Vast lowland swamp forests covered the land. These lush forests further decreased the carbon dioxide in the atmosphere and enriched it with oxygen. The oxygen rich environment resulted in the emergence of _____. Insect species grew into great sizes. Centipedes were about two meters long, cockroaches reached one meter, and dragonflies had wingspans of about one meter.

• arthropods

• sea scorpion

• Prehistoric sharks

• lobefins

• gigantic creatures