Origin of Humans (Homo sapien)

The Origin of Birds Based on fossils, most paleontologists agree that the ancestor of birds was a type of small, feathered dinosaur. Recent fossil discoveries in China support this hypothesis of a close relationship between dinosaurs and birds. They show dinosaurs from more than 100 million years ago that are covered from tail to beak with structures that appear to be feathers.

The most famous bird fossil, Archaeopteryx, represents an animal that lived about 150 million years ago during the Mesozoic Era. Unlike modern birds, Archaeopteryx had clawed forelimbs, teeth, and a long tail with vertebrae—all reptilian characteristics. In fact, if the feathers had not been preserved in the fossil, Archaeopteryx probably would have been identified as a small dinosaur.

Origin and General Characteristics of Mammals The oldest fossils of mammals (class Mammalia) date back about 220 million years to the early Mesozoic Era. The fossil evidence supports the hypothesis that mammals evolved from mammal-like reptiles called therapsids.

The early mammals co-existed with dinosaurs throughout the Age of Reptiles.. About 65 million years ago, mammals diversified as they adapted to habitats and ecosystem roles that were vacated due to the extinction of dinosaurs.

Objectives Describe some general characteristics of primates. Describe two key evolutionary adaptations in the origin of humans. Explain two hypotheses for the origin of modern humans from Homo erectus.

Afrotheria Fossils of humans and human ancestors provide a connection to the past. The fossil record indicates that modern humans have a relatively recent history that began in Africa. In only a short time, humans have become the most numerous and widespread large animal on Earth.

A Closer Look at Primates The mammals called primates (order Primates) include lemurs, monkeys, apes, and humans.primates There are approximately 300 primate species living today

Primates, even those that now live on the ground, retain adaptations for living in the trees. For example, primates have flexible shoulder joints, which make it possible to swing from one branch to another. The hands of primates can hang onto branches and manipulate food. Many primate species have sensitive fingers with nails instead of claws.

Primates have binocular vision, meaning that the two eyes are close together on the front of the face. The fields of vision of the two eyes overlap—like a pair of binoculars— enhancing depth perception, which is the ability to estimate how far away an object is. This is a valuable adaptation for animals that maneuver in trees. The excellent eye-hand coordination of primates is also important for maneuvering in the treetops.

Mammals devote more energy to caring for their young than do most other vertebrates. And primates are among the most attentive parents of all mammals. Most primates have single births and nurture their offspring for a long time.

Biologists divide living primates into two groups. One group includes lorises, galagos, and lemurs. With the exception of some lemurs, most primates of this group are nocturnal tree- dwellers. Many have claws rather than nails.

The second primate group includes tarsiers, New World monkeys, Old World monkeys, apes, and humans. Tarsiers are limited to Southeast Asia, are nocturnal, and have flat faces with large eyes. All New World monkeys—those found in the Americas—are tree-dwelling. Some have prehensile tails (grasping tails) that function as an extra appendage for swinging through trees.prehensile tails

Early Hominids Mammals that are more closely related to humans than to any other species family are called hominids (family Hominidae).hominids Only one species of hominid exists today— Homo sapiens, modern humans. However, a diversity of hominids lived in the past, including the probable ancestors of H. sapiens

Two important developments in the evolution of hominids were upright posture (leading to two-legged walking) and enlargement of the brain.

Walking Upright Fossilized bones of a hominid species named Australopithecus afarensis provide the oldest evidence of upright posture and walking on two feet, called bipedalism.bipedalism

Enlarged Brain An enlarged brain relative to body size in hominids first appears in fossils from East Simple handmade stone tools are sometimes found with the larger-brained fossils of this species, which has been named Homo habilis ("handy man").

About 2 million years after first walking upright, the ancestors of modern humans were using their skilled hands and big brains to invent tools (Figure 26-22). These tools probably enhanced the ability to hunt, gather, and scavenge for food.

Figure By comparing skulls like these, scientists can trace the evolution of the enlarged brain through hominid history.

The Origin of Modern Humans The fossil record suggests that H. erectus gave rise to diverse hominids in different regions. For example, the Neanderthals were H. erectus descendants who lived in what are now Europe, the Middle East, and Asia. The oldest fully-modern fossils of H. sapiens— skulls and other bones that look like those of today's humans—are about 100,000 years old and were discovered in Africa. Similar fossils almost as old have been discovered in Israel

There are two hypotheses for the origin of fully-modern humans from earlier hominids. According to the "multiregional hypothesis," fully-modern humans evolved in several parts of the world from Homo erectus descendants that spread from Africa over 1.5 million years ago. Occasional inbreeding among regional populations could explain how H. sapiens evolved as a single species.

But according to the second hypothesis, all regional descendants of H. erectus, outside of Africa, including the Neanderthals, became extinct. According to this view, fully-modern humans evolved from a population of H. erectus descendants that remained in Africa. These fully-modern humans spread out of Africa less than 100,000 years ago, "replacing" the then-extinct descendants of the much earlier H. erectus migration.."

Comparisons of DNA samples from human populations around the world today support a very recent split from a common ancestor. This molecular evidence favors the "replacement hypothesis