A HISTORY OF LIFE

1. A HISTORY OF LIFE

2. Origin and Evolution of the Universe • Entire universe arose about 15 billion years ago. • The solar system started taking shape about 5 billion years ago. • The sun ignites as condensed hydrogen gas atoms begin fusing to form helium. • The earth starts to coalesce and cool about 4.5 billion years ago. • The early earth was rich in water, silica, metals, heavier molecules, and other rocky components.  • Early oceans became salty as carbonates, Mg+2, K+, and other ions accumulated as acids (e.g., H2CO3, HNO3, H2SO4) interacted with water and inorganic matter.   • Atmosphere arose as the earth formed and as out-gassing occurred from within the earth, CO (carbon monoxide), CO2, H2S (hydrogen sulfide), CH4 (methane), and NH3 (ammonia).  The characteristics of the early atmosphere is based, in part, on what planetary astronomers have discovered from other planets in the solar system. • Conditions on earth, before life began - Refer to Figures 18-6 and 18-7 on pages 380 and 382 in the textbook • First protobiont appears about 3.5 billion years ago. 

3. Origins of Primeval Life • In the 1930s, Alexander I. Oparin from Russia and J. B. S. Haldane from England, proposed that life arose under certain conditions that allowed for abiotic formation of organic compounds.  • Oparin and Haldane assumed that three conditions had to exist: • Anoxic atmosphere  (presence of free oxygen would tend to interfere with chemical reactions that transform simple organic molecules into complex ones) • Precursor molecule supply had to be in abundance • A source of  Energy existed to start and keep the process going (electrical, chemical, light, etc.)

4. Stanley Miller’s Experiment • In 1953, Stanley Miller (in Urey's Lab) tried to replicate such conditions in the laboratory to see what would happen.  • In a sterile vessel containing no O2, Miller mixed a molecule supply (i.e., H2, CH4, H2O, and NH3), a source of energy (i.e., an electric spark - other folks have used UV light), and modulated the temperature from 0-100 C over the course of one week.  • At the end of the week, Miller collected samples of the "primordial soup" and analyzed it.  • Constituents of the soup did not contain life, but it did contain complex organic macromolecules, including amino acids, nucleosides, polyphosphates, and others. • Refer to Figures 18-8 and 18-9 on pages 384 in the textbook.

6. The Protobiont the first living cell • How it formed, no one really knows.  • Some properties associated with living cells can arise from the interactions between organic macromolecules, water, and the environment.  • Most probably, the transition from a non-living complex system of macromolecules to a living organism somehow occurred by a yet undetermined means in accordance to the natural laws of chemistry and physics.  • Due to the relative complexity of a living cell, even a single-celled prokaryote, some scientists have ventured the hypothesis that life could not arise through abiotic (non-living) processes in the time scale (about 1 billion years) that it is thought to have occurred.  • The panspermia hypothesis suggests that life came to earth from some external source.  • The late Francis Crick, in his 1981 book, Life Itself, proposed that life was brought to earth by intelligent extraterrestrials.  • The recent suggestion that meteorites from Mars might contain fossilized remains of ancient organisms has continued to fuel the debate (refer to Figure 18-10 on pages 385 in the textbook). 

7. What happened after life began? • Early life forms were probably unicellular and resemble modern day prokaryotes.  After life got started, it probably existed in an anaerobic environment.  As more complex biochemical pathways evolved, such as photosynthesis, waste products produced of such processes (i.e., oxygen) were toxic to existing organisms. • Geological Evidence • uranite - UO2 precipitates in stream beds.  Early atmosphere was probably < 1% O2 .  Geologic record indicates uranite started to accumulate about 2 billion years ago. • iron oxide or rust suddenly starts to accumulate in rock that are 2-3 billion years old • Biochemical Evidence • Early attempts of cellular respiration, early stages of this process do not require oxygen (glycolysis, fermentation), but later stages require oxygen (oxidative respiration).

8. The origin of eukaryotes and the origin of membrane-bound organelles • Lynn Margulis (1967) has been a strong advocate for the Endosymbiont hypothesis. • This hypothesis proposes that mitochondria and chloroplasts became incorporated into cytoplasm of eukaryotes through the symbiosis of larger cells with with bacteria (mitochondria and flagella) and cyanobacteria (chloroplasts) - refer to Figure 18-16 on page 392 in the textbook. • Prokaryotes, mitochondria, and chloroplasts possess similar genomes; each type contain a naked circular loop of DNA. • ribosomes produced by prokaryotes, mitochondria, and chloroplasts are similar in size and structure and smaller than those found in the cytoplasm of eukaryotic cells • inner membranes of mitochondria and chloroplasts are similar to the plasma membranes of bacteria. • mitochondria, chloroplasts, and prokaryotes reproduce asexually through binary fission. • antibiotics that inhibit protein synthesis in bacteria also do the same to mitochondria and chloroplasts but not to inhibit protein synthesis mediated by the nucleus

9. Early Eukaryotes About 2.5 billion years ago, a billion years after the origin of the first protobionts, eukaryotes evolved and proliferate.  • We would recognize many of these forms as being similar to modern day amoebae, protozoa, and unicellular green algae.  • Multicellular life forms do not appear in the fossil record until about 0.6 bya during a period of vast change on the face of life.

10. Cambrian explosion - "Life's Big Bang" occurred about 600 million years ago • Evidence of this preserved as fossils in the Burgess Shale in British Columbia, Canada. • Animal Phyla in the Burgess Shale include:  Porifera, Brachiopoda, Arthropoda, Echinodermata, Hemichordata, and Chordata (each phylum represents a different body plan). • Other phyla present in fossil record are extinct.

11. Major evolutionary trends • Over the past 600 million years there has been an explosion of animal, plant, and fungal diversity. • A transition from being unicellular, filamentous, or forming small colonies of cells toward becoming multicellular. • greater complexity • development of organs and organ systems • changes in reproductive strategies • development of adaptations to exist in and exploit a terrestrial environment • greater interdependence/more interactions among different species

12. IF THE HISTORY OF LIFE WERE PLACED ON A 24 HOUR SCALE (Keep in mind that these times are approximate) • Midnight - first protobiont or life forms come into existence (approx. 3.5 billion years ago) or midnight.  Prokaryotes rule!  Evolution of photosynthesis eventually leads to accumulation of  toxic levels of oxygen -  leads to mass extinction. • 10:17 a.m. - first eukaryotes (e.g., unicellular organisms - protozoan-like). • 8:18 p.m. - trilobites and other aquatic multicellular organisms appear in earth's oceans (Cambrian explosion). • 8:34 p.m. - first vertebrates (e.g., marine and fresh-water fish). • 9:15 p.m. - land plants first appear. • 9:36 p.m. - winged insects take to the land and air. • 10:34 p.m.- mass extinction. • 11:18 p.m. - dinosaurs rule. • 11:19 p.m. mass extinction (approximately 75 % of all life goes extinct). • 11:33 p.m. (approximately 65 mya) - adaptive radiation of birds and mammals. • 11:59 p.m. - Humans arrive on the scene (200,000 years ago). • 11:59:56 p.m.- Birth of human civilization.

13. HUMAN EVOLUTION • Mammals- share characteristics with other mammals such as:  vertebrate with a spinal chord, skeleton, skull housing a large brain, ability to give birth to live offspring, mammary glands, hair or fur, and common ancestry. • Humans differ from other mammals: • Teeth • canines - for tearing and piercing (most carnivores) • incisors - nip and cut food (rodents) • premolars with cusps - grinding and crushing (horses) • molars with cusps - grinding and crushing (cattle) • Early and more primitive mammals have 66 teeth; modern mammals have 44; humans have 32. • The incidence of Wisdom teeth appear to be diminishing (natural selection?) in human populations (in Central Europe, one or more wisdom teeth are missing in 19% of population.  Wisdom teeth or third molars are common among Native Americans, but not among Africans). • Offspring have an extended period of learning. • Humans have an overall larger brain size. • Humans possess behavioral flexibility

14. TRENDS IN PRIMATE EVOLUTION • Change in overall skeletal structure and mode of locomotion - bipedalism (able to move on 2 appendages for extended periods of time; with minimum energy loss) -refer to Figure 17-21 and 1725 on page 368 and 371 in the textbook . • Modification of hands - humans can cup hands and possess a opposable thumb - refer to Figure 17-23 on page 370 in the textbook • Less reliance on sense of smell and more reliance on sense of daytime and color vision, and depth perception. • Change in dentition - primates moved from eating insects to more fruits and vegetables to becoming omnivorous - adaptation of teeth is probably caused by natural selection, so that the kinds of teeth best able to accommodate a particular diet become enhanced over time - refer to Figure 17-23 on page 369 in the textbook. • Brain expansion - more elaborate. • Gorilla 600 cm3 • Humans 1350 cm3 • Higher intelligence may have resulted from tool making, need for better memory, or to increase ability to anticipate jumps (from branch to branch) or throws (weapons and spears).  • Behavioral and cultural evolution- ability to learn and mimic behavior.  ex. language.

15. CULTURAL EVOLUTION • Biogeographic Origin of Humans Africa. • The movement to different types of environments may have influenced cultural evolution.  • Cultural evolution tends to be Lamarckian because changes can be acquired and passed on.  • Examples are language, tool making, technology, and domestication of plants and animals.  • The transition from humans making a living as hunter/gathers to an agriculturally-based civilization started about 12-14,000 years ago, based on archeological evidence.

16. ANCESTORS OF MODERN HOMO SAPIENS • Mitochondrial "Eve": determined through lineage coalescence; all human descending from a very small population @ 150,000 to 200,000 years ago.  • These dates are inferred based on the fact that mitochondrial DNA is maternally inherited and on assumption that mutations in this DNA happen at consistent rate.  • Thus far the fossil evidence from South & Eastern Africa tends to support these findings.

17. The family tree for primates • Prosimians - lemuroids; lemurs, lorises, etc. • Tarsioids - tarsioids; tarsiers • Anthropoids - ceboids; new world monkeys • Cercopithecoids - old world monkeys; prehensile tails • Hylobatids - gibbons, siamang • Pongids - orangutan, gorilla, chimps • Hominids - humans and their most recent ancestors

18. Humans and Chimpanzees • Morphologically humans and apes are distinct from one another.  • Based on molecular data, isozyme polymorphisms and sequences of mitochondrial and genomic DNA, humans and apes, in particular, chimpanzees are quite similar.  • The A, B, and O blood type system for humans and chimpanzees are the same. • Humans and chimpanzees share 52 % of the same gene alleles.  • Nucleic acid differences are even less, 1.1 percent difference.  • Should humans be classified as Pan sapiens instead of Homo sapiens, or should chimpanzees be called Homo troglodytes instead of Pan troglodytes?  • Was the common ancestor to humans and chimpanzees separated by the Great Rift Valley in Africa, leading to allopatric speciation?  • Humans probably evolved in response to changing environmental conditions as forests gave way to savannas.  Some evidence supports this hypothesis, but it is far from conclusive.

19. Did Humans Arise from Apes?

20. Are human descended from apes? • The answer is NO! Based on scientific data, it appears that humans and apes share a common ancestor whose lineage diverged 15-20 million years ago. • Apes and humans probably share a common ancestor. • The common ancestor may have been ape-like, but it was not an actual ape in the modern sense.