Origin and Evolution of Vertebrates

Chapter 29: Origin and Evolution of Vertebrates

Lobe-Fins (Sarcopterygii)

  • Originated in the Silurian period.

  • Characterized by muscular pelvic and pectoral fins.

  • Used for swimming and 'walking' underwater across substrates.

Lineages of Lobe-Fins

  • Three lineages remain:   - Coelacanths   - Lungfishes   - Tetrapods

  • Coelacanths were believed to have become extinct 75 million years ago until a living specimen was captured off the coast of South Africa in 1938.

Tetrapods

  • Definition: Tetrapods are gnathostomes that have limbs.

  • Significant evolutionary event: The adaptation of fins from some lobe-fins into the limbs and feet that characterize tetrapods.

Derived Characters of Tetrapods

  • Key adaptations include:   - Four limbs with digits.   - Neck, allowing separate head movement.   - Fusion of the pelvic girdle with the backbone.   - Absence of gills, except in some aquatic species.   - Ears for detecting airborne sounds.

Origin of Tetrapods

  • Tiktaalik, nicknamed a "fishapod":   - Exhibited both fish and tetrapod characteristics such as:     - Fins, gills, lungs, and scales.     - Ribs for breathing air and support.     - A neck and shoulders.     - Fin structure resembling tetrapod limbs.

  • Tiktaalik could prop itself on its fins but was not capable of true walking.

  • First tetrapods appeared approximately 365 million years ago.

Evolutionary Timeline of Vertebrates

  • Notably includes:   - Lungfishes   - Eusthenopteron   - Panderichthys   - Tiktaalik   - Acanthostega   - Tulerpeton   - Amphibians and amniotes, with key periods: Silurian, Devonian, Carboniferous, Permian (with corresponding timelines in millions of years).

Amniotes

  • Definition: Tetrapods adapted for life on land that have a terrestrially adapted egg.

  • Two major living groups include:   - Reptiles (including birds)   - Mammals

  • Key derived character: The amniotic egg, which protects the embryo with membranes.

Importance of the Amniotic Egg

  • Major adaptation for terrestrial life.

  • Amniotic eggs, particularly in reptiles and some mammals, often have a shell.

  • Other terrestrial adaptations for amniotes:   - Relatively impermeable skin.   - Use of the rib cage for lung ventilation.

Structure of the Amniotic Egg

  • Contains several extraembryonic membranes:   - Chorion: involved in gas exchange.   - Yolk: provides nutrients.   - Albumen: protective layer.   - Amnion: encapsulates the embryo.   - Yolk sac: nutrient store.   - Allantois: involved in waste management and gas exchange.   - Shell: protects the egg.

Classification of Chordates

  • Major categories include:   - Echinodermata   - Cephalochordata   - Urochordata   - Myxini (hagfish)   - Petromyzontida (lampreys)   - Chondrichthyes (cartilaginous fish)   - Actinopterygii (ray-finned fish)   - Actinistia (lobe-finned fish)   - Dipnoi (lungfishes)   - Amphibia   - Reptilia   - Mammalia (includes significant groups like monotremes, marsupials, and eutherians).

Characteristics of Mammals

  • Definition: Mammals are amniotes with hair and mammary glands for milk production.

  • General features of mammals include:   - Mammary glands   - Hair   - High metabolic rate due to endothermy.   - Larger brain relative to body size compared to other vertebrates.   - Differentiated teeth.

Evolution of Mammals

  • By the early Cretaceous, three lineages emerged:   - Monotremes   - Marsupials   - Eutherians

  • Significant adaptive radiation for mammals did not occur until after the Cretaceous.

Monotremes

  • Small group of egg-laying mammals, including:   - Echidnas   - Platypus.

Marsupials

  • Include:   - Opossums   - Kangaroos   - Koalas.

  • Early development occurs within a placenta; offspring are born in an early developmental stage and complete development in a pouch (marsupium).

Eutherians (Placental Mammals)

  • Compared to marsupials, have a more complex placenta.

  • Young develop fully within the uterus, connected to the mother by the placenta.

  • Conflicting molecular and morphological data exist regarding the timing of eutherian diversification.

Derived Characteristics of Humans

  • Distinct features that set humans apart from other apes:   - Upright posture and bipedal locomotion.   - Larger brains with capabilities for language, symbolic thought, artistic expression, and tool use.   - Reduced jawbones and jaw muscles.   - Shorter digestive tract.

Genomic Similarity

  • The genomes of humans and chimpanzees are 99% identical.

  • Changes in regulatory genes can have significant effects on development.

Study of Human Origins: Paleoanthropology

  • Hominins (formerly hominids) are more closely related to humans than to chimpanzees.

  • Approximately 20 species of extinct hominins have been discovered.

Timeline of Hominin Evolution

  • The oldest fossil evidence dates back 6.5 million years, showing small brains and increasing bipedalism.

  • Common misconceptions include:   - Early hominins were chimpanzees (incorrect: they shared a common ancestor).   - Human evolution is a linear process leading directly to Homo sapiens (incorrect: it includes many branches and coexisting species).

Bipedalism

  • Emerged about 1.9 million years ago, facilitating energy-efficient long-distance travel in arid environments.

Tool Use

  • Oldest evidence of tool use: cut marks on animal bones, dated to 2.5 million years old.

  • Indicates that tool use may have originated before the evolution of larger brains.

Early Homo

  • Earliest fossils attributed to the genus Homo:   - Homo habilis, dated between 2.4 and 1.6 million years, associated with stone tools (name means "handy man").

Significant Hominin Species

  1. Homo ergaster:    - First fully bipedal, large-brained hominid.    - Existed between 1.9 and 1.5 million years ago.    - Decreased sexual dimorphism compared to ancestors.

  2. Homo erectus:    - Originated in Africa by 1.8 million years ago.    - First hominin known to have migrated out of Africa.

  3. Neanderthals (Homo neanderthalensis):    - Resided in Europe and the Near East from 350,000 to 28,000 years ago.    - Thick-boned with large brains; evidence of burial practices and tool-making.    - Genetic analysis shows gene flow between Neanderthals and Homo sapiens.

  4. Homo sapiens:    - Emerged in Africa by 195,000 years ago.    - All living humans descend from these African ancestors.    - Oldest fossils outside Africa date to about 115,000 years, located in the Middle East.

Migration and Discovery of New Hominin Species

  • Human migration to the New World occurred before 15,000 years ago.

  • Fossils found in Indonesia (2004) categorized as Homo floresiensis, indicating diversity in hominin lineage.

Overall Timeline of Hominin and Vertebrate Evolution

  • Overview of several key species through time, from early hominins to modern humans and their contemporaries, highlighting the branching evolution rather than a linear progression.