Neogene

The Neogene is a geologic period and system that lasted 20.45 million years, from the end of the Paleogene Period 23.04 million years ago to the start of the present Quaternary Period 2.58 million years ago. It is the second period of the Cenozoic and the eleventh period of the Phanerozoic. The Neogene is divided into two epochs: the earlier Miocene and the later Pliocene.

During this time, mammals and birds continued to evolve into the forms we know today. The first humans, Homo habilis, appeared in Africa near the end of this period. There were also some important changes in the positions of continents. One major event was when North America and South America connected at the Isthmus of Panama late in the Pliocene. This changed ocean currents, cutting off warm water from the Pacific to the Atlantic and leaving only the Gulf Stream to carry heat to the Arctic Ocean.

The world's climate became cooler throughout the Neogene. This cooling led to a series of large glaciations, or times when ice covered much of the land, in the Quaternary Period that followed. The term "Neogene" was created in 1853 by an Austrian scientist named Moritz Hörnes.

Divisions

The Neogene is divided into two main parts called epochs. The earlier part is the Miocene and the later part is the Pliocene.

The Pliocene Epoch has two ages: the Piacenzian Age and the Zanclean Age. The Miocene Epoch has six ages: the Messinian Age, Tortonian Age, Serravallian Age, Langhian Age, Burdigalian Age, and Aquitanian Age. Different areas of the world may use other names for these ages.

Paleogeography

During the Neogene, the continents were almost in the same places as they are today. The land bridge called the Isthmus of Panama formed, connecting North and South America. The Indian subcontinent kept moving into Asia, creating the tall mountains known as the Himalayas. As sea levels dropped, new paths called land bridges appeared between Africa and Eurasia, and also between Eurasia and North America.

Climate

The global climate during this time became more seasonal and continued to get drier and cooler. This trend started in an earlier time period called the Paleogene.

In the early part of this period, called the Early Miocene, the world was relatively cool. Later, during the Middle Miocene, Earth experienced a warm period known as the Middle Miocene Climatic Optimum. This warmth was caused by large volcanic activity in what is now the Columbia River Basalt Group. After this warm time, the climate cooled again in the Late Miocene, and ice began to build up at the poles.

During the Pliocene, the world warmed up once more in a period called the Pliocene Warm Interval. This warm time had carbon dioxide levels similar to today and is studied to understand possible future climate changes. By the end of this period, the Earth began to cool and the first big glaciations of the current Ice Age started.

Flora and fauna

During this time, plants and animals started to look much more like they do today. Some ancient reptiles, like the group called Choristodera, disappeared early on, and other reptiles such as Langstonia and Barinasuchus also vanished. The oceans were home to huge predators like Megalodon and Livyatan, and many shark species disappeared about 19 million years ago.

Mammals and birds were the main land animals, evolving in many shapes and sizes to fit different places. In North America, hoofed animals like ungulates started moving faster, probably because the land became more open. The first early humans, known as hominins, showed up near the end of this period.

Plants also changed a lot. About 20 million years ago, some conifer and cycad trees began to grow more kinds. As the climate cooled, tropical plants were replaced by trees that lose their leaves in winter, and grasslands spread out. This helped grasses grow more, and animals like horses, antelope, and bison evolved to eat them. During the cooler time called the Pliocene, big ice age animals like mammoths and woolly rhinoceros lived. With less CO₂ in the air, a special kind of plant called C₄ plants became common in grasslands. Flowers like Asteraceae (which include daisies) also spread a lot. Fossils of Eucalyptus trees have been found from the Miocene in New Zealand, even though these trees are not native there today.

Disagreements

Scientists have sometimes disagreed about when the Neogene Period ended. Traditionally, it ended at the close of the Pliocene Epoch, just before the start of the Quaternary Period.

Some scientists wanted to include ongoing time—the Quaternary—within the Neogene. Others thought the Quaternary should stand alone as a separate period. This disagreement comes from the fact that as we look at more recent times, it becomes easier to divide them into smaller pieces, and the rocks from these times are often better preserved.

An organization of scientists called the International Commission on Stratigraphy once suggested making the Quaternary a smaller part of the Neogene. Another group, the International Union for Quaternary Research, suggested ending the Neogene at 2.58 million years ago and starting the Quaternary then instead. After discussions, they agreed to make the Quaternary its own period beginning 2.58 million years ago. This means the Neogene Period ends at that same point.