Ch 25 ~ History of Life on Earth

The Origin of Life 1st fossil evidence 3.5 BYA (Earth ~4.6 BY old) 4 stages 1 – abiotic synthesis of organic molecules 2 – formation of macromolecules 3 – formation of protobionts 4 – formation of self- replicating molecules

Experimental evidence for the origin of life: Stanley Miller Heated water (H2O), methane (CH4), ammonia (NH3), hydrogen (H2), and electrical charge Amino acids were produced Others determined RNA may have been the first macromolecule with enzyme activity

Follow-up: Which gas was least abundant in Earth’s early atmosphere, prior to 2 BYA? Oxygen Carbon dioxide Methane Water vapor Ammonia oxygen

Protobionts Molecules derived abiotically that are surrounded by a crude membrane 2 key traits: Accurate reproduction Metabolism The first genetic material was probably RNA because it carries genetic info (n.t. sequence) and has enzymatic (catalytic) activity. The switch to a DNA-based genome probably occurred because DNA is more stable and less error-prone than RNA. Protobionts are not known to use DNA to program protein synthesis!!!

Origin of Unicellular Prokaryotes: 3.5-3.9 BYA (3:00) Many prokaryotes were photosynthetic CYANOBACTERIA, which resulted in production of atmospheric OXYGEN, paving the way for facultative and obligate anaerobes (oxygen appears at about 5:30) Stromatolite: layered rocks formed by prokaryotes that bind sediment films together

Origin of Eukaryotes: 2.1 BYA (7:00) Membrane infoldings eventually surrounded genetic material to form nucleus and also other components of endomembrane system. Later, engulfing aerobic prokaryote gave rise to origin of mitochondria, while engulfing photosynthetic prokaryote gave rise to origin of chloroplast.

Origin of Multicellularity: 1.2 BYA (8:00) Confined to only a few areas due to several Ice Ages Probably originated through cooperation of cells that each adopted specialized tasks

Follow-up: If natural selection in some environment favored the production of daughter cells that were genetically different from mother cells, one should expect selection for: I – polynucleotide polymerases with low mismatch error rates II – polynucleotide polymerases without proofreading capability III - polynucleotide polymerases with proofreading capability IV - polynucleotide polymerases with high mismatch error rates II and IV

Cambrian Explosion: 535 MYA (10:00) Extensive diversification of animal kingdom Occurrence of predation features Claws, claspers defense – spines, body armor Coincided with melting that ended the series of Ice Ages known as “snowball Earth”

Colonization of land: 500 MYA (10:20) Fungi, plants, & animals move onto land Associated with critical adaptations to terrestrial life

Appearance of Homo sapiens: 195,000 YA (11:59 and 59 sec)

Factors affecting evolution of species, I Continental drift Important in biogeography and climate changes

Factors affecting evolution of species, II Mass extinctions: Permean, 251 MYA Cretaceous, 65 MYA How/why do you think mass extinctions have such an impact on species?

Factors affecting evolution of species, III Adaptive Radiation Periods of evolutionary change in which species are adapting to new ecological roles

Slight genetic differences can produce major morphological differences in species HOMEOTIC GENES Homeotic genes (HOX) control overall body plan – anterior head, midsection thorax, posterior abdomen with limbs at specified positions occurring in specific numbers. The position of homeotic genes along the chromosome is aligned with the position of these structures in the organism. Therefore duplication or rearrangement of these genes can alter the overall body plan of the organism – which can be an advantageous, neutral, or harmful mutation

Evolution is NOT goal-oriented It all comes down to two things: survival and reproduction Exaptation: use of existing structures for new purposes Paedomorphosis: using juvenile structures in adult organisms The organisms that survive and reproduce are those who have traits that are best suited to the environment – and the environment may change at any moment. Exaptation – nat sel can only act on EXISTING phenotypes…opposable thumbs can also be used for written language, a feature that makes humans distinct from our close relatives the chimps. Paedomorphosis - Axolotls are salamanders that retain gills and other structures due to genetic changes from their ancestors. Use of juvenile structures is a beneficial adaptation.