CHAPTER 25 ORIGIN OF LIFE ON EARTH Past organisms were very different from those now alive The fossil record shows macroevolutionary changes over large time scales, for example: The emergence of terrestrial vertebrates The impact of mass extinctions The origin of flight in birds © 2011 Pearson Education, Inc.

Figure 25.1 Figure 25.1 What does fossil evidence say about where these dinosaurs lived?

So where did the first organic molecules come from? Miller-Urey and others demonstrated that organic materials can be produced from inorganic molecules under certain circumstances.

MILLER-UREY EXPERIMENT Conditions of Early Atmosphere of Earth Before Life Products of Experiment Flask with H, methane, ammonia, H20 (no free oxygen! O2) Spark (to represent lightning) Some amino acids, and cytosine and uracil

Figure 25.2a Under Volcanic Conditions (green) the results were even more notable than the original experiment (orange) 20 200 10 100 Number of amino acids Mass of amino acids (mg) Figure 25.2 Amino acid synthesis in a simulated volcanic eruption. 1953 2008 1953 2008

“Bubble Theory” RNA monomers have been produced spontaneously from simple molecules Small organic molecules polymerize when they are concentrated on hot sand, clay, or rock This can form a type of “bubble” AKA : Protocell, vesicle, micelle Provides an enclosed protected area for reactions to take place © 2011 Pearson Education, Inc.

Protocells Replication and metabolism are key properties of life and may have appeared together Protocells may have been fluid-filled vesicles with a membrane-like structure In water, lipids and other organic molecules can spontaneously form vesicles with a lipid bilayer First cells may have formed on sand or clay in warm-shallow water. © 2011 Pearson Education, Inc.

Which came first? DNA, RNA, or Protein DNA first- Probably not. What does DNA require in order to replicate? RNA first Not only can RNA replicate, but it can sometimes act like an enzyme (ribozyme). Protein first Not only can proteins perform countless functions, but they can also replicate (prions).

Self-Replicating RNA and the Dawn of Natural Selection The first genetic material was probably RNA, not DNA RNA molecules called ribozymes have been found to catalyze many different reactions For example, ribozymes can make complementary copies of short stretches of RNA © 2011 Pearson Education, Inc.

The First Eukaryotes The oldest fossils of eukaryotic cells date back 2.1 billion years Eukaryotic cells have a nuclear envelope, mitochondria, endoplasmic reticulum, and a cytoskeleton The endosymbiont theory proposes that mitochondria and plastids (chloroplasts and related organelles) were formerly small prokaryotes living within larger host cells An endosymbiont is a cell that lives within a host cell © 2011 Pearson Education, Inc.

heterotrophic eukaryote Ancestral photosynthetic Figure 25.9-3 Plasma membrane Cytoplasm DNA Ancestral prokaryote Nucleus Endoplasmic reticulum Photosynthetic prokaryote Mitochondrion Nuclear envelope Aerobic heterotrophic prokaryote Figure 25.9 A hypothesis for the origin of eukaryotes through serial endosymbiosis. Mitochondrion Plastid Ancestral heterotrophic eukaryote Ancestral photosynthetic eukaryote

Concept 25.2: The fossil record documents the history of life The fossil record reveals changes in the history of life on Earth Sedimentary rocks are deposited into layers called strata and are the richest source of fossils © 2011 Pearson Education, Inc.

Stromatolite fossils are the remains of prokaryotes Figure 25.4 Present Dimetrodon Rhomaleosaurus victor 100 mya 1 m 175 Tiktaalik 0.5 m 200 270 300 4.5 cm Hallucigenia Coccosteus cuspidatus 375 400 1 cm Figure 25.4 Documenting the history of life. Dickinsonia costata 500 525 2.5 cm Stromatolites 565 600 Fossilized stromatolite 1,500 3,500 Tappania Stromatolite fossils are the remains of prokaryotes

How Rocks and Fossils Are Dated Sedimentary strata reveal the relative ages of fossils The absolute ages of fossils can be determined by radiometric dating A “parent” isotope decays to a “daughter” isotope at a constant rate Each isotope has a known half-life, the time required for half the parent isotope to decay © 2011 Pearson Education, Inc.

Remaining “parent” isotope Time (half-lives) Figure 25.5 Accumulating “daughter” isotope Fraction of parent isotope remaining 1 2 Remaining “parent” isotope Figure 25.5 Radiometric dating. 1 4 1 8 1 16 1 2 3 4 Time (half-lives)

Concept 25.3: Key events in life’s history include the origins of single-celled and multicelled organisms and the colonization of land The geologic record is divided into the Archaean, the Proterozoic, and the Phanerozoic eons The Phanerozoic encompasses multicellular eukaryotic life The Phanerozoic is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic © 2011 Pearson Education, Inc.

Geologic Time Scale Page 515 Table 25.1 Geologic Time Scale Page 515 Table 25.1 The Geologic Record

Cambrian Explosion Concurrent events that may have lead to an overwhelming increase in the number of species Hox Genes Predator / Prey Relationships (defined by cephalization- having a head) Sequestration of carbon dioxide into algae and fossilization (what is happening to that carbon today?) © 2011 Pearson Education, Inc.

Time (millions of years ago) Figure 25.10 Sponges Cnidarians Echinoderms Chordates Brachiopods Annelids Molluscs Figure 25.10 Appearance of selected animal groups. Arthropods PROTEROZOIC PALEOZOIC Ediacaran Cambrian 635 605 575 545 515 485 Time (millions of years ago)

EXTINCTION Major boundaries between geological divisions correspond to extinction events in the fossil record © 2011 Pearson Education, Inc.

Mass Extinctions The fossil record shows that most species that have ever lived are now extinct Extinction can be caused by changes to a species’ environment At times, the rate of extinction has increased dramatically and caused a mass extinction Mass extinction is the result of disruptive global environmental changes © 2011 Pearson Education, Inc.

(families per million years): Figure 25.15 1,100 25 1,000 900 20 800 700 15 600 (families per million years): Total extinction rate Number of families: 500 10 400 300 5 200 Figure 25.15 Mass extinction and the diversity of life. 100 Era Paleozoic Mesozoic Cenozoic Q Period E O S D C P Tr J C P N 542 488 444 416 359 299 251 200 145 65.5

A number of factors might have contributed to these extinctions Intense volcanism in what is now Siberia Global warming resulting from the emission of large amounts of CO2 from the volcanoes Reduced temperature gradient from equator to poles Oceanic anoxia (low oxygen) from reduced mixing of ocean waters © 2011 Pearson Education, Inc.

Is a Sixth Mass Extinction Under Way? Scientists estimate that the current rate of extinction is 100 to 1,000 times the typical background rate Extinction rates tend to increase when global temperatures increase Data suggest that a sixth, human-caused mass extinction is likely to occur unless dramatic action is taken (by 2050) Did you have any plans for the second half of this century? © 2011 Pearson Education, Inc.

Adaptive Radiations Adaptive radiation is the evolution of diversely adapted species from a common ancestor Adaptive radiations may follow Mass extinctions The evolution of novel characteristics The colonization of new regions © 2011 Pearson Education, Inc.

Evolutionary Novelties Most novel biological structures evolve in many stages from previously existing structures Complex eyes have evolved from simple photosensitive cells independently many times Exaptations are structures that evolve in one context but become co-opted for a different function Natural selection can only improve a structure in the context of its current utility © 2011 Pearson Education, Inc.

E: Squid (Similar to Mammal) Figure 25.26 A; Limpit (Patella) B: Some mollusks C: Nautilus D: Marine snail E: Squid (Similar to Mammal) (a) Patch of pigmented cells (b) Eyecup Pigmented cells (photoreceptors) Pigmented cells Epithelium Nerve fibers Nerve fibers (c) Pinhole camera-type eye (d) Eye with primitive lens (e) Complex camera lens-type eye Epithelium Cornea Cellular mass (lens) Fluid-filled cavity Cornea Lens Figure 25.26 A range of eye complexity among molluscs. Retina Optic nerve Optic nerve Optic nerve Pigmented layer (retina)

Page 531 Figure 25.27 Figure 25.27 The evolution of horses. Holocene Equus Pleistocene Pliocene Hippidion and close relatives 5 Nannippus Pliohippus Sinohippus Neohipparion 10 Callippus Miocene Hipparion Megahippus Hypohippus 15 Archaeohippus Anchitherium Parahippus Merychippus 20 25 Millions of years ago Miohippus Oligocene 30 Haplohippus 35 Figure 25.27 The evolution of horses. Palaeotherium Mesohippus Key Pachynolophus Grazers 40 Epihippus Browsers Propalaeotherium Eocene 45 Orohippus 50 Hyracotherium relatives 55 Hyracotherium