Big Idea 1

Ch. 26 – The Tree of Life

• Geologic history and biological history have been episodic, marked by revolutions that opened many new ways of life

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” (spherical collection of lipids proposed as a stepping-stone to the origin of life)

4. Origin of self-replicating molecules (like cells)

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

• Experiments simulating an early Earth atmosphere produced organic molecules from inorganic precursors

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

Protobionts• Small organic molecules polymerize

when they are concentrated on hot sand, clay, or rock

• 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

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

• Early protobionts with self-replicating, catalytic RNA would have been more effective at using resources and would have increased in number through natural selection

The fossil record chronicles life on Earth

• Fossil study opens a window into the evolution of life over billions of years

• Sedimentary strata reveal the relative ages of fossils

Fossils cont.

• 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

Fossils cont. (dating fossils)

• The absolute ages of fossils can be 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

The Geologic Record• By studying rocks and fossils at many

different sites, geologists have established a geologic record of Earth’s history

• 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

*There have been 5 major extinctions*You do not need to know the names and

dates of these extinctions for the exam

• Lesser extinctions mark boundaries of many periods within each era

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

• 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

• Mass extinctions provided life with unparalleled opportunities for adaptive radiations into newly vacated ecological niches

ChicxulubcraterYucatán

Peninsula

LE 26-8

Millions of years ago600 500 400 300 200 100

Permian massextinction

100

80

60

40

20

0

Extinction rate

Cretaceousmass extinction

02,500

2,000

1,500

1,000

500

0

Neo

gene

Prot

eroz

oic

eon

Cam

bria

n

Ord

ovic

ian

Silu

rian

Dev

onia

n

Carb

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Perm

ian

Tria

ssic

Jura

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Cret

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Pale

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Paleozoic MesozoicCeno-zoic

Num

ber o f families (

)Extin

ction

rate

()

Number oftaxonomic

families

• A clock analogy can be used to place major events in the context of the geological record

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

• Prokaryotes were Earth’s sole inhabitants from 3.5 to about 2 billion years ago

Photosynthesis and the Oxygen Revolution

• 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

• 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

Eukaryotic cells arose from symbioses and genetic exchanges between prokaryotes

• The oldest fossils of eukaryotic cells date back 2.1 billion years

• Among the most fundamental questions in biology is how complex eukaryotic cells evolved from much simpler prokaryotic cells

• 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

LE 26-13

Plasmamembrane

Cytoplasm

DNA

Ancestralprokaryote

Endoplasmic reticulum

Nuclear envelope

Infolding ofplasma membrane

Engulfing of aerobicheterotrophicprokaryote

Nucleus

Cell with nucleusand endomembranesystem

Mitochondrion

Engulfing ofphotosyntheticprokaryote insome cells

Plastid

Mitochondrion

Ancestralheterotrophiceukaryote

Ancestralphotosynthetic eukaryote

• Key evidence supporting an endosymbiotic origin of mitochondria and plastids:– Similarities in inner

membrane structures and functions

– Both have their own circular DNA

Eukaryotic Cells as Genetic Chimeras

• Endosymbiotic events and horizontal gene transfers may have contributed to the large genomes and complex cellular structures of eukaryotic cells

• Eukaryotic flagella and cilia may have evolved from symbiotic bacteria, based on symbiotic relationships between some bacteria and protozoans

The Earliest Multicellular Eukaryotes

• 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

• 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

LE 26-15

150 mm 200 mm

Two-celled stage of embryonicdevelopment (SEM)

Later embryonic stage(SEM)

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 “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 (jellyfish) and Porifera (sea sponge), are somewhat older, dating from the late Proterozoic

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

LE 26-17

500

EarlyPaleozoicera(Cambrianperiod)

Mill

ions

of y

ears

ago

542

LateProterozoiceon

Spon

ges

Cnid

aria

ns

Echi

node

rms

Chor

date

s

Brac

hiop

ods

Anne

lids

Mol

lusc

s

Arth

ropo

ds

New information has revised our understanding of the tree of life

• Early classification systems had two kingdoms: plants and animals

• Robert Whittaker proposed five kingdoms: Monera, Protista, Plantae, Fungi, and Animalia

• Molecular data have provided insights into the deepest branches of the tree of life

• The five kingdom system has been replaced by three domains: Archaea, Bacteria, and Eukarya

• Each domain has been split into kingdoms