Chapter 26: The Tree of LifeOverview: Changing Life on a Changing Earth• Life is a continuum extending from the earliest organisms to the great variety of speices alive today.• Organisms interact with their environments.◦ Geological events that alter environments change the course of biological history.▪ When glaciers recede and the land rebounds, marine creatures can be trapped in what gradually become freshwater lakes.▪ Populations of organisms trapped in these lakes are isolated from parent populations, and may evolve into new species.◦ Living things change the planet they inhabit.▪ The evolution of photosynthetic organisms released oxygen into the air, with a dramatic effect on Earth’s atmosphere.◦ The emergence of Homo sapiens has changed the land, water, and air at an unprecedented rate.• Geologic history and biological history have been episodic, marked by what were in essence revolutions that opened many new ways of life. • Historical study of any sort is an inexact discipline that depends on the preservation, reliability, and interpretation of past records.◦ The fossil record of past life is generally less and less complete the further into the past we delve.◦ Fortunately, each organism alive today carries traces of its evolutionary history in its molecules, metabolism, and anatomy.◦ Still, the evolutionary episodes of greatest antiquity are generally the most obscure.Concept 26.1: Conditions on early Earth made the origin of life possible• Most biologists now think that it is credible that chemical and physical processes on Earth produced simple cells.• According to one hypothetical scenario, there were four main stages in this process:1. The abiotic (nonliving) synthesis of small organic molecules, such as amino acids and nucleotides(monomers).2. The joining of monomers into polymers.3. The packaging of these molecules into “protobionts,” droplets with membranes that maintained a distinct internal chemistry.4. The origin of self-replicating molecules that eventually made inheritance possible.• The scenario is speculative but does lead to predictions that can be tested in laboratory experiments.Synthesis of Organic Compounds on Early Earth• Earth and the other planets in the solar system formed about 4.6 billion years ago, condensing from a vast cloud of dust and rocks surrounding the young sun.• It is unlikely that life could have originated or survived in the first few hundred million years after theEarth’s formation.◦ The planet was bombarded by huge bodies of rock and ice left over from the formation of the solar system.◦ These collisions generated enough heat to vaporize all available water and prevent the formation of the seas.• The oldest rocks on the Earth’s surface, located at a site called Isua in Greenland, are 3.8 billion yearsold.◦ It is not clear whether these rocks show traces of life.• In the 1920s, Russian chemist A. I. Oparin and British scientist J. B. S. Haldane independently postulated that Earth’s early atmosphere had been a reducing (electron adding) environment, in whichorganic compounds could have formed from simple molecules. ◦ The energy for this organic synthesis could have come from lightening and intense UV radiation• Haldane suggested that the early oceans were a solution of organic molecules, a “primitive soup” from which life arose.• In 1953, Stanley Miller and Harold Urey tested the Oparin-Haldane hypothesis by creating, in thelaboratory, the conditions that had been postulated for early Earth.• They discharged sparks in an “atmosphere” of gases and water vapor.• The Miller-Urey experiments produced a variety of amino acids and other organic molecules.◦ Other attempts to reproduce the Miller-Urey experiment with other gas mixtures have also produced organic molecules, although in smaller quantities.• It is unclear whether the atmosphere contained enough methane and ammonia to be reducing.◦ There is growing evidence that the early atmosphere was made up primarily of nitrogen and carbon dioxide.◦ Miller-Urey-type experiments with such atmospheres have not produced organic molecules.▪ It is likely that small “pockets” of the early atmosphere near volcanic openings were reducing.• Alternate sites proposed for the synthesis of organic molecules include submerged volcanoes and deep-sea vents where hot water and minerals gush into the deep ocean.◦ These regions are rich in inorganic sulfur and iron compounds, which are important in ATP synthesis by present-day organisms.Extraterrestrial Sources of Organic Compounds• Some of the organic compounds from which the first life on Earth arose may have come from space.• Researchers are looking outside of Earth for clues about the origin of life.◦ Evidence is growing that Mars was relatively warm for a brief period, with liquid water and an atmosphere rich in carbon dioxide.◦ During that period, prebiotic chemistry similar to that on early Earth may have occurred on Mars.◦ Did life evolve on Mars and then die out, or did dropping temperatures and a thinning atmosphereterminate prebiotic chemistry before life evolved?◦ Liquid water lies beneath the ice-covered surface of Europa, one of Jupiter’s moons, raising the possibility that Europa’s hidden ocean may harbor life.◦ Detection of free oxygen in the atmosphere of any planets outside our solar system would be strongly suggestive of oxygenic photosynthesis.Abiotic Synthesis of Polymers• The abiotic origin hypothesis predicts that monomers should link to form polymers without enzymes and other cellular equipment.• Researchers have produced polymers, including polypeptides, after dripping solutions of monomers onto hot sand, clay, or rock.◦ Similar conditions likely existed on early Earth at deep-sea vents or when dilute solutions of monomers splashed onto fresh lava.Protobionts• Life is defined by two properties: accurate replication and metabolism.◦ Neither property can exist without the other.• DNA molecules carry genetic information, including the information needed for accurate replication.◦ The replication of DNA requires elaborate enzymatic machinery, along with a copious supply of nucleotide building blocks provided by cell metabolism.• Although Miller-Urey experiments have yielded some of the nitrogenous bases of DNA and RNA, they have not produced anything like nucleotides.◦ Thus, nucleotides were likely not part of