Theory of Evolution Bio. Standard 3.4.1

Key Concepts What was the early atmosphere like? How do experiments suggest first “cells” may have evolved? How did early conditions affect the type of organisms that developed?

How did life on Earth begin? The current scientific thinking is based on a relatively small amount of evidence, so it is likely that these ideas will change Earth is about 4.6 billion years old and started as pieces of cosmic debris that collided and stuck together; these and subsequent collisions produced enough heat to melt the entire planet

The molten Earth STRATIFIED (layered) according to density, while radioactive elements in the interior generated enough heat to keep the interior molten The least dense elements, hydrogen and nitrogen, along with hydrogen cyanide, carbon dioxide, carbon monoxide, hydrogen sulfide, and water, formed the first atmosphere

About 3.8 billion years ago, Earth’s surface cooled enough for water to remain a liquid; rain produced oceans, which were brown with dissolved iron, that covered much of the globe

Atoms do not assemble themselves into complex organic molecules or living cells on Earth today for a number of reasons 1. oxygen is very reactive and would destroy many kinds of organic molecules not protected within cells 2. as soon as organic molecules appeared, something (bacteria) would probably eat them

MILLER AND UREY suggested how mixtures of the organic compounds necessary for life could have arisen from simpler compounds present on a primitive Earth

While Miller and Urey’s simulations of Earth’s early atmosphere were not accurate, similar experiments based on more current knowledge have also produced organic compounds, including cytosine and uracil

About 200 to 300 million years after liquid water could exist on early Earth, cells similar to modern bacteria were common-but how might these cells have originated?

PROTEINOID MICROSPHERES can form under certain conditions when large organic molecules form tiny bubbles While microspheres are not living cells, they do have selectively permeable membranes and a simple means of storing and releasing energy, suggesting that structures similar to these may have acquired more and more characteristics of living cells over time

A number of discoveries about RNA suggests that RNA may have existed before DNA 1. RNA can help DNA replicate 2. some RNA sequences process messenger RNA after transcription 3. some RNA sequences catalyze chemical reactions 4. some RNA sequences can grow and duplicate themselves How simple RNA-based forms of life led to the system of DNA-directed protein synthesis that exists now is still unclear

MICROFOSSILS-microscopic fossils-of prokaryotes that resemble modern bacteria have been found in rocks more than 3.5 billion years old; these must have evolved in the absence of oxygen

Over time, photosynthetic bacteria evolved and began producing oxygen, as evidenced by formation of iron oxide, which precipitated to the ocean floor As oxygen gas accumulated in Earth’s atmosphere, concentrations of methane and hydrogen sulfide decreased, and the ozone layer formed The rise of oxygen in the atmosphere drove some life forms to extinction, while other life forms evolved new, more efficient metabolic pathways that used oxygen for respiration; anaerobic organisms were forced into a few airless habitats

About 2 billion years ago, prokaryotic cells began evolving internal cell membranes; the end result was ancestral eukaryotic cells Then other prokaryotes began entering these ancestral eukaryotic cells and began living inside the larger cells in a symbiotic relationship

ENDOSYMBIOTIC THEORY suggests that eukaryotic cells formed from a symbiosis among several different prokaryotic organisms Ex. prokaryotes that performed cellular respiration became mitochondria; prokaryotes that performed photosynthesis became chloroplasts

Evidence of endosymbiotic theory includes 1. DNA similar to bacterial DNA in chloroplasts and mitochondria 2. ribosomes in chloroplasts and mitochondria similar to bacterial DNA 3. chloroplasts and mitochondria reproduce by binary fission when cells containing them divide during mitosis

Some time after eukaryotic cells arose, those cells began to reproduce sexually, which greatly sped the process of evolution-but how? Most prokaryotes reproduce asexually, which restricts genetic variation

Sexual reproduction shuffles genes so that the probability of more favorable combinations of genes is increased, increasing the chances of evolutionary change in a species due to natural selection A few hundred million years after evolution of sexual reproduction, multicellular organisms evolved from single-celled organisms, which allowed for a great increase in diversity

Evidence of Common Ancestry Fossil evidence Biochemical similarities Anatomical structures (homologies)

Common Ancestry The evolutionary relationship between many organisms can be traced back to a common ancestor. A common ancestor is an individual from which two or more related species could have evolved. With the passage of time, organisms change and diverge from their common ancestor to form new species

Biochemistry DNA, RNA, the genetic code and protein synthesis are similar in all organisms. The greater the genetic and molecular similarity between species, the closer their common ancestor. Humans and chimpanzees have 98% of their genes in common. The remaining 2% is what distinguishes these two species from each other.

Diabetics can use insulin from cows and pigs because insulin from these animals is almost identical to human insulin. In addition, hemoglobin in humans, which has almost 600 amino acids, is almost identical to hemoglobin in all other vertebrates. This similarity in chemical structure demonstrates that all vertebrates can be traced back to a common ancestor.

Anatomical Structures

Embryo Similarities

Vestigial Organs structures or organs that seem to serve no useful function ex. human tailbone. The vestigial tailbone in humans is homologous to the functional tail of other primates.

Macroevolution Large-scale evolutionary patterns and processes that occur over long periods of time