THINK ABOUT IT
At first glance, fishes, amphibians, reptiles, birds, and mammals appear to be very different. Yet, all are members of the phylum in which we ourselves are classified—phylum Chordata.
Lesson Overview
Chordate Evolution and Diversity
Origins of the Chordates
What are the most ancient chordates?
Embryological studies suggest that the most ancient chordates were related to the ancestors of echinoderms.
Lesson Overview
Chordate Evolution and Diversity
Nonvertebrate Chordates
Adult tunicates (subphylum Urochordata) look more like sponges than us. They have neither a notochord nor a tail. But their larval forms have all the key chordate characteristics.
For example, the small, fishlike lancelets (subphylum Cephalochordata) live on the sandy ocean bottom.
Lesson Overview
Chordate Evolution and Diversity
Jawless Fishes
The earliest fishes appeared in the fossil record about 510 million years ago. They had no true jaws or teeth, and their skeletons were made of cartilage.
Lesson Overview
Chordate Evolution and Diversity
Sharks and Their Relatives
Early fishes also evolved paired pectoral (anterior) and pelvic (posterior) fins. Paired fins offered more control of body movement, while tail fins and powerful muscles gave greater thrust. The evolution of paired fins and tail fins launched the adaptive radiation of the class Chondrichthyes: the sharks, rays, and skates.
Lesson Overview
Chordate Evolution and Diversity
Bony Fishes
Another group of ancient fishes evolved skeletons made of true bone, launching the radiation of the class Osteichthyes, the bony fishes.
- Ray-finned Fish
- Lobe-finned Fish
- Lungfishes
Lesson Overview
Chordate Evolution and Diversity
Amphibians
Amphibians are vertebrates that also, with some exceptions, require water for reproduction, breathe with lungs as adults, have moist skin with mucous glands, and lack scales and claws.
Lesson Overview
Chordate Evolution and Diversity
Reptiles
Reptiles were the first vertebrates to evolve adaptations to drier conditions. A reptile is a vertebrate with dry, scaly skin, well-developed lungs, strong limbs, and shelled eggs that do not develop in water.
Lesson Overview
Chordate Evolution and Diversity
Birds
Birds are reptiles that regulate their internal body temperature (endothermy.) They have an outer covering of feathers; strong yet lightweight bones; two legs covered with scales that are used for walking or perching; and front limbs modified into wings.
Lesson Overview
Chordate Evolution and Diversity
Mammals
Characteristics unique to mammals include mammary glands in females that produce milk to nourish young, and hair. Mammals also breathe air, have four-chambered hearts, and regulate their internal body temperature. By the beginning of the Cenozoic Era, three major groups of mammals had evolved—monotremes, marsupials, and placentals.
Lesson Overview
Chordate Evolution and Diversity
Lesson Overview
Chordate Evolution and Diversity
Evolution of Primates
What are the major evolutionary groups of primates?
Primates in one of these groups look very little like typical monkeys. This group contains the lemurs and lorises. The other group includes tarsiers and the anthropoids, the group that includes monkeys, great apes, and humans.
Lesson Overview
Chordate Evolution and Diversity
Anthropoids split into two groups around 45 million years ago, as the continents on which they lived moved apart.
The New World monkeys are found in Central and South America.
Members of this group live almost entirely in trees. They have long, flexible arms that enable them to swing from branches.
New World monkeys also have a long, prehensile tail that can coil tightly enough around a branch to serve as a “fifth hand.”
The other anthropoid branch, which evolved in Africa and Asia, includes the Old World monkeys and great apes.
Old World monkeys spend time in trees but lack prehensile tails.
Lesson Overview
Chordate Evolution and Diversity
Hominine Evolution
What adaptations enabled later hominine species to walk upright?
The skull, neck, spinal column, hip bones, and leg bones of early hominine species changed shape in ways that enabled later species to walk upright.
Lesson Overview
Chordate Evolution and Diversity
Hominine Evolution
The evolution of bipedal, or two-footed, locomotion was very important, because it freed both hands to use tools.
The hominine hand evolved an opposable thumb that could touch the tips of the fingers, enabling the grasping of objects and the use of tools.
Hominines evolved much larger brains. Most of the difference in brain size results from an expanded cerebrum, which is, as you recall, the “thinking” part of the brain.
Lesson Overview
Chordate Evolution and Diversity
Relatives Versus Ancestors
The hominine fossil record includes seven genera—Sahelanthropus, Orrorin, Ardipithecus, Australopithecus, Paranthropus, Kenyanthropus, and Homo—and at least 20 species.
Lesson Overview
Chordate Evolution and Diversity
The Road to Modern Humans
What is the current scientific thinking about the genus Homo?
If you look at the Hominine Time Line, you can see that many species in our genus existed before our species, Homo sapiens, appeared. Furthermore, at least three other Homo species existed at the same time as early humans.
Lesson Overview
Chordate Evolution and Diversity
Out of Africa—But When and Who?
Researchers agree that our genus originated in Africa and migrated from there to populate the world.
Some current hypotheses about when hominines first left Africa and which species made the trip are shown in the figure.
Lesson Overview
Chordate Evolution and Diversity
Modern Homo sapiens
Neanderthals and H. sapiens lived side by side in the Middle East for about 50,000 years.
Later, both groups moved into Europe, where they coexisted for several thousand years.
For the last 24,000 years, however, Homo sapiens has have been Earth’s only hominine.
Why did Neanderthals disappear? Did they interbreed with H. sapiens? No one knows for sure.
Lesson Overview
Chordate Evolution and Diversity