The History of Life on Earth Chapter 25

Objectives Conditions that led to origin of life on earth The history of life as seen in fossils Key events include origin of single-cellular and multi-cellular organisms and the development of terrestrial life The effects of continental drift, mass extinctions, and adaptive radiation on groups of animals Developmental genes can radically change body forms Understand why evolution is NOT goal oriented.

Early earth Abiotic synthesis of amino acids and nucleotides Formation of macrocmolecules (proteins and nucleic acids) Packaging these molecules into protobionts Origin of self replicating molecules that led to inheritance 3.5 billion years is the earliest fossil evidence of microorganisms Scientists hypothesize that chemical and physical processes could produce simple cells.

Synthesis of organic compounds Oparin-Haldane hypothesis—Early earth was a reducing atmosphere that led to synthesis Miller and Urey tested the hypothesis which yielded amino acids Demonstrated that abiotic synthesis was possible Analysis of meteorites show presence of AAs Synthesis of macromolecules may have been initiated by formation of AA polymers In the 1920s Oparin and Haldane proposed that the energy to produce the organic compounds could come from lightning or intense UV radiation. In 1953 Stanley Miller and Harold Urey of U of Chicago tested the Oparin-Haldane hypothesis. Others have tested it as well with different recipes for the atmosphere. The major part of the hypothesis may not be correct in that the atmosphere of earth may not have been reducing nor oxidizing. IT is thought that pockets near volcanic openings were reducing and that first organic compounds formed near underwater volcanic vents. The abiotic synthesis of macromolecules may have occurred through the dripping of amino acid solutions on hot sand, rock or clay. This forms a AA polymer that could act as a weak catalyst for other reactions.

Protobionts Protobionts are abiotically created molecules surrounded by a membrane. They can engage in simple reproduction and metabolism The can maintain an internal environment different than the external environment. Liposomes can spontaneously organize in water from lipids and organic molecules

Self replicating RNA RNA catalysts are called ribozymes Protobionts with RNA were more successful The development of DNA provided a more stable molecule for genetic information

Key events in earth’s history Earliest evidence of life comes from fossilized stromatolites, which are cyanobacteria that bind sediment together in layers. Stromatolites can be found in warm, shallow and salty bays. The evidence is from 3.5 billion years ago which means that single celled organisms were around earlier. The photosynthetic processes used by cyanobacteria likely produced oxygen that first dissolved in water, oxidized iron and eventually escaped into the atmosphere. Prokaryotes that evolved to metabolized oxygen became abundant because of the efficiency of aerobic metabolism. The higher the O2 levels the more complex cells could become because enough O2 could diffuse across the cell membranes (prokaryotes require 1% of atmospheric O2 and larger eukaryotes require 2-3% of current atmospheric O2)

The First Eukaryote Eukaryotic cells added a nuclear envelope, endoplasmic reticulum, mitochondria and other internal structures (Golgi) and they ave a cytoskeleton, which allowed them to change shape and engulf other cells. Enodsymbiosis is a model that is used to explain the origin of some of theses structures and organelles. An endosymbiont is an organism that is engulfed by a host cell. Plastids and mitochondria are endosymbionts. They have systems that are homologous to prokaryotes.

The First Eukaryote

Origin of multicellularity First evidence of multicellular organisms (algae) ~1.2 billion years The fossil record indicates that the first major diversification of multicellularity was after a thaw ~565 million years Ediacaran biota The Cambrian explosion Prior to the Cambrian explosion animals were soft bodied and probably herbivores. After the explosion there was evidence of claws and other features that suggested predation on other animals as well as features that provided defense. The origin of many animal groups may have extended tens of years before the Cambrian explosion.

Colonization of land ~ 500 mya Adaptations developed to live on land Plants produced waterproof coating and a vascular system for internal transport Early plants had no roots or leaves Fungi followed plants Arthropods are the most abundant land animals Tetrapods arrived ~365mya Our species arrived 195,000 years ago

Rise and fall of organisms Continental drift Formation of mountains Oceanic plates usually slide below terrestrial plates Scientists infer the past location of continents by examining the direction of magnetic north which is recorded in the rock as it is formed

Continental drift Alters habitats Reroutes ocean currents Changes weather patterns Promotes allopatric speciation Helps explain why fossils in two different regions can be the same When pangaea was formed the oceans became deeper and there was less shallow waters. The center of pangaea was cold and dry

Mass Extinctions Permian extinction Cretaceous extinction The majority of species that have ever lived are now extinct. Physical factors such as a change in the temperature of the ocean can cause extinction. Extinction is always occurring but when the rate increases dramatically then we call them mass extinctions. The Permian extinction clained 96% of marine species and latered life in the ocean. 8 of 27 orders of insects became extinct. This extinction was though to occur in possibly a few thousand years to 5 million years. There were also volcanic eruptions in Siberia and it is thought that carbon dioxide increased to the level where it increased the temp of the earth 6oC. This led to reduced mixing of water because temperature differences did not vary. This in turn reduced oxygen levels in the oceans and may have led to the marine extinctions. The Cretaceous extinction led to half of all marine life species extinguished and many terrestrial animals and plants. This is when the dinosaurs became extinct. It is thought that this mass extinction was precipitated by the earth’s collision with a comet or asteroid. This would have led to blocking of the sun and a change in climate.

Cretaceous extinction The common ground 1. There was global climatic change; the environment changed from a warm, mild one in the Mesozoic to a cooler, more varied one in the Cenozoic. The cause of this climate change, and the speed at which it proceeded, are the major concerns of both schools of thought. 2. There were acute changes to the atmosphere as well that led to a longer term global greenhouse effect, which reduced sunlight and increased temperatures.. These changes may have been the result of a massive terrestrial disturbance, which threw up soot into the air, causing short term acid rain, emission of poisonous gases, and cooling (similar to a nuclear winter). 3. Many organisms; both marine and terrestrial, vertebrate and invertebrate; went extinct likely because of the climate change. 4. At or near the K-T boundary in several places around the globe, we have a thin layer of clay with an unusually high iridium (a rare metal similar to platinum) content. This may be evidence for the dust cloud in #2 above. Two theories The "intrinsic gradualists“ posit that the extinction was gradual and took several million years to occur. They have two hypotheses that appear to support one another There was a great deal of volcanic activity that could produce weather changes as well as be the source of the iridium, since iridium is found in the earth’s core. There was movement of the land masses—plate tectonics. Oceans were retreating from the land over a long period of time. This produced weather changes that may have led to certain lineages becoming extinct. Movement of land masses leads to volcanic activity. The "extrinsic catastrophists“ believe that the extinction was caused by an extraterrestrial and sudden event. The Alvarez Hypothesis supports this view. A large meteor or a group of meteors bombarded the earth, throwing up dust clouds that radically changed the climate. Some scientists have suggested that groups of asteroids or comets from Oort’s cloud periodically bombard the earth every 26 million years. Is there a 6th big extinction? The current rate of extinction is 100 to 1000 times the typical background rate.

Consequences of mass extinctions Evolutionary lineages disappear Reduction in the diversity of an ecosystems Increase in predators Arising of adaptive radiations

Mass extinction and predators Is there a 6th big extinction? The current rate of extinction is 100 to 1000 times the typical background rate. Predators increased following each extinction, although the recovery took 100-150 million years.

Adaptive radiation An organism’s movement into a variety of different environments or exploitation of a variety of different food sources leads to adaptive radiation. The mass extinction of dinosaurs gave way to adaptive radiation of mammals 65 million years ago.

Mammalian adaptive radiation Extinction of the dinosaurs led to adaptive radiation of mammals. Mammals were small and noturnal even 120 million years after they appeared, 180 million years ago. When dinosaurs disappeared, with the exception of birds, mammals filled those ecological niches.

The Silversword Alliance Dubautia laxa 1.3 million years KAUAI 5.1 million years MOLOKAI MAUI OAHU 3.7 million years LANAI Argyroxiphium sandwicense HAWAII 0.4 million years All of these plants evolved from an ancestral tarweed that arrived 5 million years ago from North America. They are part of the silversword alliance. In Hawaii each island is diverse in terms of elevation, rainfall, soil, and as a result many different types of ecosystems evolved. Dubautia waialealae Dubautia scabra Dubautia linearis

Exaption Using a trait that evolved for one purpose for another purpose. The lightweight honeycombed bones of early non-flying birds were taken advantage of by birds that fly. Feathers were initially developed for camouflage, courtship, or thermoregulation. Later they developed for flight. Karel Liem, “Evolution is like modifying a machine while it is running.”

Evo-devo Genes that control development have had a profound effect on evolution Effect growth rates of particular body parts Controls timing of the emergence of particular structures Controls spatial pattern of particular structures

Evolution and development Genes control the rate, timing, and spatial pattern of development. Proportioning of the body is known as allometric growth, which alters proportions during development.

Evolution and development Varying the rate of growth of different body regions leads to morphological changes.

Heterochrony and salamanders Foot growth gets turned off later in ground dwelling salamanders. Paedomorphosis Axolotl salamander. A change in a single gene locus was probably sufficient to produce paedomorphosis in this salamander. Paedomorphosis---the sexually mature stage juvenile features because reproductive development was accelerated compared to somatic development.

Changes in spatial pattern Homeotic genes, such as Hox genes control spatial organization of body features. Chicken leg bud Region of Hox gene expression Zebrafish fin bud

Hypothetical vertebrate ancestor (invertebrate) with a single Hox cluster First Hox duplication Hypothetical early vertebrates (jawless) with two Hox clusters Second Hox Vertebrates (with jaws) with four Hox clusters Most invertebrates have one set of Hox genes that direct development of major body parts. 520 mya a duplication mutation may have provided the genetic material that gave rise to vertebrates. In early vertebrates the genes took on a new role—the development of the backbone. 425 myo a second duplication mutation yielded 4 clusters which allowed development of greater structural complexity (jaws and limbs?) The vertebrate Hox contains duplicates of genes found in the single Hox complex of invertebrates and in the same order on the chromosome and they direct the sequential development of the same body regions. Scientists infer that the vertebrate Hox genes are homologous to the Hox genes found in invertebrates

New developmental genes Ubx suppresses formation of legs in insects but not in crustaceans

Gene regulation Gene regulation can change in different cell types which can lead to changes in the development of different structures in different organisms.

Evolved complexity Mollusc eye evolution Slit shell Pleurotomania Pigmented cells (photoreceptors) Pigmented cells Epithelium Nerve fibers Nerve fibers Patch of pigmented cells Eyecup Mollusc eye evolution Slit shell Pleurotomania Limpet Patella Fluid-filled cavity Cellular fluid (lens) Cornea Epithelium Optic nerve Pigmented layer (retina) Optic nerve Pinhole camera-type eye Eye with primitive lens Marine snail Murex Nautilus Eyes have a single evolutionary origin. Incremental modification led to the development of different eyes that respond to light Evidence for evolutionary divergence comes from phylogenetic analysis of the genes that control the development of eyes. Cornea Lens Retina Optic nerve Squid Loligo Complex camera-type eye

Hippidion and other genera Recent Equus Hippidion and other genera Pleistocene Nannippus Pliohippus Hipparion Neohipparion Pliocene Sinohippus Megahippus Callippus Archaeohippus Merychippus Miocene Anchitherium Hypohippus Parahippus Miohippus Oligocene Branched evolution of horses. Note this is not a progressive trend. Browsers fed on brushes and trees and grazers fed on grasses. Grazers developed increased size, a single toe, and modified teeth for grazing. Horses are the result of a number of adaptive radiations. Mesohippus Paleotherium Epihippus Propalaeotherium Eocene Pachynolophus Orohippus Key Grazers Hyracotherium Browsers