1AST 309L Part IV.Evolution, Convergence, Intelligence2Fusion vs. Fission313N radioactive decay: idea of half-life4Uranium radioactive decay5Useful radioactive isotopes for age determinations6Establishing biological nature of fossils: stomatolites (below), 12C/13C ratio, …7Tree of life from comparisons of small subunit ribosomal RNA (shared by all living things)sequences (same as in text). See notes for method.8Tree of life--a few branch ages indicated9Tree of life with thermophiles and hyperthermophiles in bold10Tree of life showing photosynthesizing bacteria, hyperthermophiles, and methanogenic archeans11Tree of life showing Giardia--one of earliest eukaryotes?12History of life on Earth13Prokaryote-Eukaryote cells14Prokaryote-Eukaryote difference in complexity again15One model for endosymbiotic origin of eukaryotes--viral invasion16We see mostly Eukaryotic life forms (below) but by far the most numerous and varied (bynumber, species, diversity, metabolic activities, habitat, …) are the bacteria. In many viewseukaryotes are nothing but vehicles for bacterial symbiosis.17Filling up the atmosphere with oxygen due to photosynthesis: evidence from banded ironformations18Approximate evolution of the composition of Earth’s atmosphere19A crucial evolutionary “event”: the Precambrian/Cambrian transition and the appearance oflarger, more complex organisms in a short period of time20The environment mattered too: This shows how Earth’s history alternated between glacial andinterglacial periods back to 400,000 yr (“Snowball Earth” episode probably occurred a little earlier)21Throughout Earth’s history the distribution of land mass has been changing radically: thisleads to large changes in climate regulation that has affected life (see textbook)22Examples of very old microfossils and modern counterparts23Cyanobacterial microfossils: Age ~ 1.5 billion years24Eukaryotic microfossils: Ages 1.5 to 0.57 billion years25Ediacaran (Precambrian/Cambrian transition) fossils26Sudden increase in complexity after Precambrian/Cambrian transition ~ 550 Myr ago27Summary of some major events since 1200 Myr ago28Diversity continued to “explode” after the Precambrian/Cambrian transition29Review of major evolutionary events• Earliest biomarkers ~ 3500 to 3800 Myr.• Lateral (horizontal) transfer of genetic information between bacteria rampant• Photosynthesis ~ 3000 Myr ago? (recent evidence pushes back to very earlytimes, ~ 3600 Myr)• “Oxygen holocaust”--atmosphere fills up with O2 after crust saturated (~ 2000Myr). But this also probably made energy production more effective foreukaryotes (text, p. 127).• Origin of eukaryotes by endosymbiosis (mitochondria, chloroplasts,…)--establishes “modularity” as major structural feature of complex design. (Fig. 5.11in text) Notice similarity in timing for atmospheric oxygen and eukaryotes• Meiotic (sexual) reproduction--increases diversity, prevents mutational meltdown(~ 1000 Myr?), allows development of larger genomes?30Major evolutionary events (continued)• Snowball Earth episode (?)--complete glaciation (~ 600-800 Myr?). [See outsidereadings for details. Also text, p. 106 in ch. 4]• Cambrian explosion--sudden appearance of large and complex life forms (545Myr). Without this event, no SETI. [Outside readings and pp. 128-129 in text]• Colonization of land by plants and fungi, leading to land animals (~ 500 Myr)--nopredators yet! [pp. 129-130 in text. Note importance for SETI].• Mass extinctions: several major, many minor, between ~ 500 Myr and 65 Myr(and some would say today). 65 Myr extinction ⇒ “dinosaur extinction” due tolarge asteroid impact (also called K-T event). No mammal dominance without this?See pp. 130-137 in text for good discussion + class notes.• Hominids to earliest homo sapiens: 6 to 0.2 Myr (will discuss separately). Sec.5.6 in text + class notes + outside readings on intelligence)31The Sun’s brightening could lead to a second (and final) microbial era in the future: Ifduration of “animal era” is so brief, chances of communication (SETI) would be very small.32So major question for SETI is: Do we expect some or most of these developments to occurelsewhere? Was evolution “convergent?” We especially want to know if this is the case forcomplex traits like “intelligence.”• Could point to MANY examples (see class notes and pictures to follow): flight, pouchesw,sonar, eyes, jet propulsion, even social structures.• But these could be due to lateral transfer, in particular “viral transduction.”• Also, interpreting traits is tricky because of things like exaptation (see class notes)• And some traits would be very useful (adaptive) but have never arisen (e.g. cellulose-digesting enzyme in animals), or have arisen only once (the case of woodpeckers is discussedin class notes).• And there are undoubtedly processes, especially environmental processes like impacts, thatare completely unpredictable. We’ll discuss mass extinctions next.• We need to look at the genome-level processes of evolution to get any idea about whetherwe expect convergence to occur or not (next topic, beginning of Part II of notes).33Examples of convergence of adaptive traits: Swimming and marsupials/placentals34Convergence: burrowing mammals35Convergence: complex eye design36Major Mass Extinctions37Mass extinction by genera: extinction and origination38Mass extinction at 65 Myr: Almost certainly ~10 km bolide impact• Iridium-rich layer in 65 Myr sedimentary layers foundworldwide (iridium is enhanced in meteorites)• Mass of iridium consistent with impact bolide size ~10km• Where is the crater? “Smoking gun” found inChixhulub (Yucatan coast, Mexico). Age =65 Myr!39Some large impact craters notassociated with enhanced extinction• Large crater near Quebec, Canada Details of Chicxulub structure40Consequences and rates of impact of bolides with different
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