1. Chapter 26
The Tree of Life

2. Geological events that alter environments
1. How Life Began
Geological events that alter environments
Change the course of biological evolution
Conversely, life changes the planet that it inhabits
Figure 26.1

3. Conditions on early Earth made the origin of life possible Hypothesis:
Chemical and physical processes on early Earth produced very simple cells through a sequence of stages

4. Earth formed ~ 4.6 billion years ago Earth’s early atmosphere
Contained water vapor + many chemicals released by volcanic eruptions
Little or no O2  reducing atmosphere
But it did have: CO, CO2, H2, N2, H2O, S, and HCl,

5. 2. SEAS
First organic compounds may have formed near submerged volcanoes and deep-sea vents
Figure 26.3

6. Lab simulations of early Earth atmosphere:
3. Complex Molecules
Lab simulations of early Earth atmosphere:
EXPERIMENT
Miller and Urey set up a closed system in their laboratory to simulate conditions thought to have existed on early Earth. A warmed flask of water simulated the primeval sea. The strongly reducing “atmosphere” in the system consisted of H2, methane CH4), ammonia (NH3), and water vapor. Sparks were discharged in the synthetic atmosphere to mimic lightning. A condenser cooled the atmosphere, raining water and any dissolved compounds into the miniature sea.
CH4
Electrode
Water vapor
RESULTS
NH3 H2
As material circulated through the apparatus, Miller and Urey periodically collected samples for analysis. They identified a variety of organic molecules, including amino acids such as alanine and glutamic acid that are common in the proteins of organisms. They also found many other amino acids and complex, oily hydrocarbons.
Condenser
Cold
water
CONCLUSION
Organic molecules, a first step in the origin of life, can form in a strongly reducing atmosphere.
Cooled water
containing
organic
molecules
H2O
Sample for
chemical analysis
Figure 26.2

7. Some organic cmpds. may have come from space
Carbon cmpds. have been found in some of the meteorites

8. 4. Abiotic synthesis of polymers
Small organic molecules
Polymerize when they are concentrated on hot sand, clay, or rock

9. 5. Protobionts
Aggregates of abiotically produced molecules surrounded by membrane

10. Protobionts could have formed spontaneously from abiotically produced organic cmpds
e.g., small membrane-bounded droplets called liposomes form when lipids are added to water

11. Figure 26.4a, b Glucose-phosphate 20 m Phosphorylase Starch Amylase
(a) Simple reproduction. This lipo- some is “giving birth” to smaller liposomes (LM).
(b) Simple metabolism. If enzymes—in this case, phosphorylase and amylase—are included in the solution from which the droplets self-assemble, some liposomes can carry out simple metabolic reactions and export the products.
Glucose-phosphate
Phosphorylase
Starch
Amylase
Maltose
Phosphate
Figure 26.4a, b

12. Complementary RNA copy
RNA molecules (ribozymes) catalyze many different reactions, including
Self-splicing
Making copies of short stretches of their own sequence
Figure 26.5
Ribozyme
(RNA molecule)
Template
Nucleotides
Complementary RNA copy 3 5

13. Early protobionts with self-replicating, catalytic RNA
used resources and increased in number through natural selection

14. As prokaryotes evolved, they exploited and changed young Earth
6. Heterotrophs
As prokaryotes evolved, they exploited and changed young Earth
Oldest known fossils are stromatolites
Rocklike structures w/ many layers of bacteria and sediment
3.5 billion years old

15. Lynn Margulis (top right), of the University of Massachussetts, and Kenneth Nealson, of the University of Southern California, are shown collecting bacterial mats in a Baja California lagoon. The
mats are produced by colonies of bacteria that live in environments inhospitable to most other life. A section through a mat (inset) shows layers of sediment that adhere to the sticky bacteria as
the bacteria migrate upward.
Some bacterial mats form rocklike structures called stromatolites,
such as these in Shark Bay, Western Australia. The Shark Bay
stromatolites began forming about 3,000 years ago. The inset shows a section through a fossilized stromatolite that is about
3.5 billion years old.
(a)
(b)
Figure 26.11a, b

16. Prokaryotes were Earth’s sole inhabitants
From 3.5 to about 2 billion years ago

17. Electron transport systems of a variety of types
Essential to early life
Some aspects that possibly precede life itself

18. Earliest types of photosynthesis
Did not produce O2
Cyanobacteria are autotrophic prokaryotes that obtain their energy and manufacture organic compounds by photosynthesis

19. Oxygenic photosynthesis
8. Oxygen Revolution
Oxygenic photosynthesis
Evolved ~ 3.5 bya in cyanobacteria
Figure 26.12

20. O2 began to accumulate in the atmosphere ~ 2.7 bya
Challenge for life
Opportunity to gain energy from light
Allowed organisms to exploit new ecosystems

21. As the ozone formed the UV light was absorbed by the atmosphere disrupting the energy available for abiotic synthesis of organic compounds.

22. 9. Eukaryotes
Eukaryotic cells arose from symbioses and genetic exchanges between prokaryotes

23. Oldest fossils of eukaryotic cells
2.1 billion years

24. Endosymbiosis
Mitochondria and plastids were formerly small prokaryotes living within larger host cells

25. Probably undigested prey or internal parasites
Figure 26.13
Cytoplasm
DNA
Plasma
membrane
Ancestral
prokaryote
Infolding of
plasma membrane
Endoplasmic
reticulum
Nuclear envelope
Nucleus
Engulfing
of aerobic
heterotrophic
Cell with nucleus
and endomembrane
system
Mitochondrion
eukaryote
Plastid
Engulfing of
photosynthetic
prokaryote in
some cells
Photosynthetic

26. Eventually host and endosymbionts would have become a single organism

27. Supporting evidence: Similar inner membrane structures and functions
Both have their own circular DNA (genes)
Own ribosomes

28. Chinese paleontologists recently described 570-million-year-old fossils
probably animal embryos
Figure 26.15a, b
150 m
200 m
(a) Two-cell stage
(b) Later stage

29. The first multicellular organisms were colonies
Collections of autonomously replicating cells
10 m
Figure 26.16

30. Some cells in the colonies
Became specialized for different functions
The first cellular specializations
Had already appeared in the prokaryotic world