1. The Tree of Life: An Introduction to Biological Diversity Chapter 26

2. Overview: Changing Life on a Changing Earth
Life is a continuum extending from the earliest organisms to the variety of species that exist today
Geological events change the course of evolution
Conversely, life changes the planet that it inhabits

3. Planet Formation

4. Planet Formation

5. Planet Formation

6. Conditions on early Earth made the origin of life possible
Chemical and physical processes on early Earth may have produced very simple cells through a sequence of stages:
1. Abiotic synthesis of small organic molecules
2. Joining of these small molecules into polymers
3. Packaging of molecules into “protobionts”
4. Origin of self-replicating molecules

7. Synthesis of Organic Compounds on Early Earth
Earth formed about 4.6 billion years ago, along with the rest of the solar system
Earth’s early atmosphere contained water vapor and chemicals released by volcanic eruptions

8. Synthesis of Organic Compounds on Early Earth
Ancient Atmospheric Gases—H2O, CH4 (methane), NH2 (ammonia)
Additional gases (common emissions of modern volcanoes)—CO, CO2, N2, H2O vapor, H2S/FeS, HCN, H2
Meteor bombardment
Lightning, heat, and UV radiation served as energy sources

9. Experiments simulating an early Earth atmosphere produced organic molecules from inorganic precursors, but such an atmosphere on early Earth is unlikely
LE 26-2
Water vapor
CH4
NH3 H2
Electrode
Condenser
Cold
water
Cooled water
containing
organic
molecules
Sample for
chemical analysis
H2O
Stanley Miller

10. Instead of forming in the atmosphere, the first organic compounds may have been synthesized near submerged volcanoes and deep-sea vents

11. Extraterrestrial Sources of Organic Compounds
Some organic compounds from which the first life on Earth arose may have come from space
Carbon compounds have been found in some meteorites that landed on Earth
Some carbonaceous chondrites have revealed the presence of 74 different amino acids, and all 5 of the cross-linking bases of RNA and DNA.

12. Mars Rock and putative microbes

13. Abiotic Synthesis of Polymers
Small organic molecules polymerize when they are concentrated on hot sand, clay, or rock
Electrical charges concentrate monomers; dehydration reactions create polymers
Sydney Fox demonstrated formation of proteinoids (abiotically synthesized polypeptides)

14. Protobionts
Protobionts are aggregates of abiotically produced molecules surrounded by a membrane or membrane-like structure
Experiments demonstrate that protobionts could have formed spontaneously from abiotically produced organic compounds
For example, small membrane-bounded droplets called liposomes can form when lipids or other organic molecules are added to water

15. Liposome Glucose-phosphate Phosphorylase Amylase Starch Maltose
LE 26-4
Glucose-phosphate
Phosphorylase
Amylase
Starch
Maltose
Phosphate
Simple metabolism
Simple reproduction
20 mm
Liposome

16. Protobionts Microspheres—made from proteinoids mixed in cool water
Liposomes—spheres made of phospholipids mixed with water (similar to phospholipid bilayer)
Coacervates—colloidal drops of polypeptides, nucleic acids, and polysaccharides
Microspheres
Liposomes

17. The “RNA World” 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, including:
Self-splicing
Making complementary copies of short stretches of their own sequence or other short pieces of RNA

18. Ribozyme
(RNA molecule) 3¢
Template 3¢
Nucleotides
Complementary RNA copy 5¢ 5¢
Early protobionts with self-replicating, catalytic RNA would have been more effective at using resources and would have increased in number through natural selection

19. First Genetic Material and Enzymes may both have been RNA

21. The fossil record chronicles life on Earth
Fossil study opens a window into the evolution of life over billions of years

22. How Rocks and Fossils Are Dated
Sedimentary strata reveal the relative ages of fossils

23. Grand Canyon

24. Index fossils are similar fossils found in the same strata in different locations
They allow strata at one location to be correlated with strata at another location

25. Determining Age of Fossils
Relative age—placement in strata layers
Absolute age—determined by radiometric dating
The magnetism of rocks can provide dating information
Magnetic reversals of the magnetic poles leave their record on rocks throughout the world

26. Ratio of parent isotope
LE 26-7
Accumulating
“daughter”
isotope
Ratio of parent isotope
to daughter isotope 1 2
Remaining
“parent”
isotope 1 4 1 8 1 16 1 2 3 4
Time (half-lives)

27. The Geologic Record
By studying rocks and fossils at many different sites, geologists have established a geologic record of Earth’s history
scamp.wr.usgs.gov/scamp/html/scg_time.html

28. The geologic record is divided into three eons: the Archaean, the Proterozoic, and the Phanerozoic
The Phanerozoic eon is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic
Each era is a distinct age in the history of Earth and its life, with boundaries marked by mass extinctions seen in the fossil record
Lesser extinctions mark boundaries of many periods within each era

30. Mass Extinctions
The fossil record chronicles a number of occasions when global environmental changes were so rapid and disruptive that a majority of species were swept away
Mass extinctions provided life with unparalleled opportunities for adaptive radiations into newly vacated ecological niches

31. LE 26-8 Millions of years ago 600 500 400 300 200 100 Permian mass
extinction 80 60 40 20
Extinction rate
Cretaceous
mass extinction
2,500
2,000
1,500
1,000
Neogene
Proterozoic eon
Cambrian
Ordovician
Silurian
Devonian
Carboniferous
Permian
Triassic
Jurassic
Paleogene
Paleozoic
Mesozoic
Ceno-
zoic
Number of families ( )
Extinction rate (
Number of
taxonomic
families

32. It may have been caused by volcanic eruptions
The Permian extinction killed about 96% of marine animal species and 8 out of 27 orders of insects
It may have been caused by volcanic eruptions
The Cretaceous extinction doomed many marine and terrestrial organisms, notably the dinosaurs
It may have been caused by a large meteor impact
NORTH
AMERICA
Chicxulub
crater
Yucatán
Peninsula

33. As prokaryotes evolved, they exploited and changed young Earth
The oldest known fossils are stromatolites, rocklike structures composed of many layers of bacteria and sediment
Stromatolites date back 3.5 billion years ago

34. Modern stromatolites in Shark Bay, Australia
Mats of photosynthetic organisms – cyanobacteria, algae and phototrophs

36. The First Prokaryotes
Prokaryotes were Earth’s sole inhabitants from 3.5 to about 2 billion years ago
Electron transport systems were essential to early life
The earliest types of photosynthesis did not produce oxygen
Oxygenic photosynthesis probably evolved about 3.5 billion years ago in cyanobacteria

37. Effects of oxygen accumulation in the atmosphere about 2
Effects of oxygen accumulation in the atmosphere about 2.7 billion years ago:
Posed a challenge for life
Provided opportunity to gain energy from light
Allowed organisms to exploit new ecosystems

38. Eukaryotic cells arose from symbioses and genetic exchanges between prokaryotes
Among the most fundamental questions in biology is how complex eukaryotic cells evolved from much simpler prokaryotic cells
The oldest fossils of eukaryotic cells date back 2.1 billion years

39. Endosymbiotic Origin of Mitochondria and Plastids
The theory of endosymbiosis proposes that mitochondria and plastids were formerly small prokaryotes living within larger host cells
The prokaryotic ancestors of mitochondria and plastids probably gained entry to the host cell as undigested prey or internal parasites
In the process of becoming more interdependent, the host and endosymbionts would have become a single organism

40. Key evidence supporting an endosymbiotic origin of mitochondria and plastids:
Similarities in inner membrane structures and functions
Both have their own circular DNA

41. Multicellularity evolved several times in eukaryotes
After the first eukaryotes evolved, a great range of unicellular forms evolved
Multicellular forms evolved also
Molecular clocks date the common ancestor of multicellular eukaryotes to 1.5 billion years
The oldest known fossils of eukaryotes are of relatively small algae that lived about 1.2 billion years ago

42. Larger organisms do not appear in the fossil record until several hundred million years later
Chinese paleontologists recently described 570-million-year-old fossils that are probably animal embryos
150 mm
200 mm
Two-celled stage of embryonic
development (SEM)
Later embryonic stage
(SEM)

43. The Colonial Connection
The first multicellular organisms were colonies, collections of autonomously replicating cells
Some cells in the colonies became specialized for different functions
The first cellular specializations had already appeared in the prokaryotic world

44. The “Cambrian Explosion”
Most of the major phyla of animals appear in the fossil record of the first 20 million years of the Cambrian period
Two animal phyla, Cnidaria and Porifera, are somewhat older, dating from the late Proterozoic
Cnidaria
Porifera

45. Molecular evidence suggests that many animal phyla originated and began to diverge much earlier, between 1 billion and 700 million years ago

46. Colonization of Land by Plants, Fungi, and Animals
Plants, fungi, and animals colonized land about 500 million years ago
Symbiotic relationships between plants and fungi are common today and date from this time

47. Colonization of Land by Plants, Fungi, and Animals
With the development of photosynthetic bacteria came the development of an atmosphere containing oxygen
UV radiation from the sun reacted with the oxygen to form the ozone layer around the earth
Protected organisms from destructive radiation, allowing them to survive on the land

48. Arthropods
~100 million years after plants/fungi covered the earth, animals could survive on land
Arthropods are believed to be the first
Have hard outer skeleton and jointed limbs
Lobsters, insects, crabs, spiders
Specific traits allowed certain animals to survive and reproduce and pass on their genes
Eryon arctiformis

49. Vertebrates Animals with backbones (endoskeletons)
The first were jawless fishes (500 mya)
430 mya jawed fishes

50. Vertebrates Amphibians –first vertebrates on land
Had lungs, allowing them to absorb oxygen from the air
Limbs believed to be derived from bones if fish fins
Strong, flexible internal skeleton allowed animals to be much larger than insects
Plethodon glutinosus: Slimy Salamander
Lysorophid amphibian

51. Vertebrates Reptiles –350 mya
Watertight skin protected from dessication
Lay eggs with shells on land
Better adapted to dry climate
Iguana

52. Vertebrates
Birds
Winged animals
Can fly
Hollow skeleton

53. Evolution of Organisms
Extinctions –death of a species
opens up niches for other species to become more abundant
Continental drift—movement of Earth’s land masses over geologic time
Contributed to geographic distribution of some species
Marsupials in Australia and South America

54. Continental Drift
The continents drift across our planet’s surface on great plates of crust that float on the hot underlying mantle
These plates often slide along the boundary of other plates, pulling apart or pushing each other

55. Eurasian Plate North American Plate Philippine Caribbean Juan de Fuca
LE 26-18
North
American
Plate
Eurasian Plate
Philippine
Indian
Arabian
Australian
Antarctic
African
Scotia Plate
South
Nazca
Pacific
Cocos Plate
Juan de Fuca
Caribbean

56. Many important geological processes occur at plate boundaries or at weak points in the plates
Volcanoes and
volcanic islands
Trench
Oceanic ridge
Oceanic crust
Subduction zone
Seafloor spreading

57. LE 26-20
By about 10 million years
ago, Earth’s youngest
major mountain range,
the Himalayas, formed
as a result of India’s
collision with Eurasia
during the Cenozoic.
The continents continue
to drift today.
By the end of the
Mesozoic, Laurasia
and Gondwana
separated into the
present-day continents.
By the mid-Mesozoic
Pangaea split into
northern (Laurasia)
and southern
(Gondwana)
landmasses.
At the end of the
Paleozoic, all of
Earth’s landmasses
were joined in the
supercontinent
Pangaea.
65.5
135
251
Millions of years ago
Cenozoic
Mesozoic
Paleozoic
North America
Eurasia
Africa
India
South
America
Madagascar
Australia
Antarctica
Laurasia
Gondwana
Pangaea
The formation of the supercontinent Pangaea during the late Paleozoic era and its breakup during the Mesozoic era explain many biogeographic puzzles

58. New information has revised our understanding of the tree of life
Molecular data have provided insights into the deepest branches of the tree of life

59. Previous Taxonomic Systems
Plantae
Fungi
Animalia
Protista
Monera
Eukaryotes
Prokaryotes
Early classification systems had two kingdoms: plants and animals
Robert Whittaker proposed five kingdoms: Monera, Protista, Plantae, Fungi, and Animalia

60. Reconstructing the Tree of Life: A Work in Progress
The five kingdom system has been replaced by three domains: Archaea, Bacteria, and Eukarya
Each domain has been split into kingdoms
serc.carleton.edu/.../creationism/mechevol.html