History of Life
Measuring the Past
Humans and all other organisms are part of the natural world
Natural processes shape the Earth and the living things that reside on the Earth
All living things are the products of evolution
Measuring the Past
Humans share features with other animals due to shared evolutionary ancestry
e.g., Our genetic code is identical to that of virtually every living organism
Same relationship between codons and amino acids
Measuring the Past
The Earth was formed about 4.6 billion years ago
This massive time frame is divided into multiple “eras”
Precambrian, Paleozoic, Mesozoic and Cenozoic
Eras divided into “periods”
Further subdivided into “epochs”
Historic time encompasses only the last 10,000 years of this time frame
Measuring the Past
The divisions between eras periods, etc., represent times of transition in life forms
Many of these transitions are due to major extinction events
e.g., The Cretaceous Extinction marks the boundary between the Cretaceous and Tertiary periods
Largely caused by the impact of a giant asteroid
Extinction of the dinosaurs
Measuring the Past
We will discuss “notable events”in the history of life on Earth
Our choice of what is “notable”is subjective
We will discuss many events in our own evolutionary past
Evolution IS NOT a march toward the formation of humans
We could just as easily discuss events in the evolutionary history of the common dandelion
19.2 How Did Life Begin?
The early Earth was very hostile
Covered with a layer of molten rock
Bombarded by comets, asteroids, etc.
Contained gasses such as methane and ammonia
Spewed from volcanoes, released from deep-sea vents, etc.
By 3.8 billion years ago, the Earth’s environment became less hostile
Black Smoker
Black Smoker Video
Black Smoker Video
How Did Life Begin?
The basic organic molecules present in life can be spontaneously assembled from methane, ammonia, and similar gases
This can be recreated in the laboratory
Similar to what is seen at Yellowstone
How Did Life Begin?
Where this assembly took place is the only question
Life may have arisen in the “prebiotic soup”of a hot water system
Life may have arisen within the sand and silt of ancient beaches
Early Life
Yellowstone Thermal Features
Yellowstone Thermal Feature
How Did Life Begin?
Life did not originate in a single step
The ability of a molecule to self-replicate is one critical feature of life
RNA was most likely the earliest self-replicating molecule
Served as the genetic material
Errors in self-replication produced genetic variation
How Did Life Begin?
Once life was established, forms of life evolved
The earliest branchings produced three domains
Bacteria, Archaea, and Eukarya
Each of these domains branched further to form multiple kingdoms, etc.
e.g., Eukarya branched into four kingdoms (Protista, Plantae, Fungi, and Animalia
The Precambrian Era
The Precambrian Era stretched from the origin of the Earth until 542 million years ago
Many notable events occurred during the Precambrian Era
Origin of life
Origin of photosynthesis
Origin of eukaryotes
Origin of multicellular life
The Precambrian Era
The Earth was formed 4.6 billion years ago
4,600 million years ago
The earliest evidence for life is 3.7 –3.8 billion years old
3,700 –3,800 million years ago
Chemical signatures of life exist in ancient rocks
The Precambrian Era
For the first 2 billion years in the history of life, all life was either archaea or bacteria
2,000 million years
The earliest life subsisted mainly on organic matter from their surroundings
This supply was limited
The Precambrian Era
Photosynthesis arose in bacteria by 3,400 million years ago
Energy from the sun was used to produce organic molecules
A large amount of energy-rich food was made available
The Precambrian Era
Cyanobacteria produced oxygen as a product of photosynthesis
Oxygen gas was virtually absent in he atmosphere until 2,400 million years ago
Production of this oxygen drastically changed the Earth’s environments
“Oxygen holocaust”
Organisms unable to adapt to this changed environment died
The Precambrian Era
Many organisms did adapt to the presence of oxygen
Many ancient bacteria were able to metabolize oxygen
The Precambrian Era
Some of these bacteria took up permanent residence in early eukaryotic cells
“Endosymbiosis”
These bacteria became mitochondria
Organelles of eukaryotic cells
Oxygen is used to harvest energy from food
The Precambrian Era
Some cyanobacteria also took up residence within eukaryotic cells
“Endosymbiosis”
These bacteria became chloroplasts
Organelles of eukaryotic cells
Perform photosynthesis
The Precambrian Era
The oxygen produced by photosynthesis reacted to form ozone
Rose through the atmosphere to form the ozone layer
Protects life from the damaging effects of ultraviolet radiation
This protection was important in allowing the colonization of the land
The Cambrian Explosion
Throughout much of the Precambrian Era, many forms of life were present
Archaea
Bacteria
Many types of protists
Early animals appeared near the end of the Precambrian Era
600 million year-old animal fossils have been found
The Cambrian Explosion
The Cambrian Period began 542 million years ago with the “Cambrian Explosion”
Lasted only approximately 6 million years
Produced incredible diversity of animal life
Many new animal forms appeared in the fossil record during the Cambrian Explosion
First fossil evidence of 35 of the 36 currently existing animal phyla
Some extinct phyla appeared then also
The Cambrian Explosion
Did the Cambrian Explosion represent an explosion of forms large and hard enough to leave fossils?
Some divergence of animal forms occurred prior to the Cambrian Explosion
What caused the Cambrian Explosion?
The rise in atmospheric oxygen is a likely cause
Larger organisms would require more oxygen
Movement onto Land:Plants First
Plant-fungi combinations were the first multicellular life to colonize land
Occurred between 460 million and 1,300 million years ago
Most modern plants have a mutually beneficial relationship with fungi
Plants supply food through photosynthesis
Fungi aid in water and mineral absorption
Movement onto Land: Plants First
Primitive land plants evolved from algae
“Bryophytes”
Represented by today’s mosses
No vascular tissue to transport water and nutrients
Cannot grow very tall
Movement onto Land: Plants First
Early vascular plants evolved from these bryophytes
Represented by today’s ferns
Possess a vascular system
Transports water and nutrients
Affords support and allows taller growth
Movement onto Land: Plants First
Some seedless vascular plants grew quite tall
These were the dominant large plants during the reign of the dinosaurs
Seed plants evolved from seedless vascular plants beginning 350 million years ago
Movement onto Land: Plants First
Gymnosperms are the living descendents of these early seed plants
e.g., Pine and fir trees
Decreased dependence on water
Sperm are packaged into pollen grains
Carried by wind instead of swimming through dew to get to the egg
700 species alive today
Movement onto Land: Plants First
Flowering plants evolved from these early seed plants beginning 165 million years ago
“Angiosperms”
Flowers aided in reproduction of these plants
260,000 species alive today
Animals Follow Plants onto the Land
Vertebrates evolved in the oceans
Gnathostomes evolved 450 million years ago
First jawed animals
Ancestral to all modern fish
New foods became available with the evolution of jaws
Animals Follow Plants onto the Land
Early fish were ray-finned
Lacked bones in their fins
Typified by walleye, etc.
Lobe-finned fish evolved from ray-finned fish
Possessed bones two pairs of fins
Lobed fins are precursors of four limbs
Amphibians evolved from lobe-finned fish
Amphibians are “tetrapods”
Possess four limbs
Animals Follow Plants onto the Land
Amphibians evolved from lobe-finned fish
Retained a major dependence on water
Amphibian literally means “double life”
Spend some of their life in the water and some of their life on land
Notably, their eggs are laid in water or other moist environments
Animals Follow Plants onto the Land
Reptiles evolved from amphibians
Reptiles produce an “amniotic egg”
Membranes within amniotic eggs supply nutrients and remove wastes
Hard outer casing protects them from drying
Amphibian ties to water were severed
Reptiles could move inland
Animals Follow Plants onto the Land
Dinosaurs evolved from one branch of reptiles about 220 million years ago
220 million years ago
Dominant vertebrate for 155 million years
Mammals arose from another branch of reptiles shortly after the dinosaurs arose
210 million years ago
Possessed fur and mammary glands
Animals Follow Plants onto the Land
Early mammals were rather small
Lived in the shadow of dinosaurs for over 100 million years
Began an adaptive radiation near the end of the Cretaceous
The Cretaceous period ended with an asteroid impact that killed the dinosaurs 65 million years ago
Mammals radiated into many recently vacated niches
Animals Follow Plants onto the Land
Primates evolved from ancestral mammals between 55 and 90 million years ago
Key characteristics of primates
Large, front-facing eyes allowing binocular vision and enhanced depth perception
Limbs with opposable first digits
Tree-dwelling existence
The Evolution of Human Beings
Humans and chimpanzees share a common ancestor approx. 6 –7 million years ago
It took more than a single speciation to give rise to humans from this common ancestor
Numerous human-like species have been discovered
The human evolution tree is more of a bush
The Evolution of Human Beings
Taxonomic grouping Hominini
The relationships between these species are not perfectly understood
Major disagreement over which are ancestors and which are “cousins”
Nearly all of the evolution of hominins occurred in East Africa
Some species migrated from Africa to other continents (e.g. Homo erectus, Homo sapiens)
The Evolution of Human Beings
Toumai fossils
Helped push root of hominin family tree back 2 million years
Oldest fossils previously found dated to 4.4 million years old
Date shortly after the divergence of human and chimpanzee lineages
May actually belong in chimp lineage
The Evolution of Human Beings
Australopithecus afarensis
Ancestral to Homo lineage
Best known through “Lucy”
Largely intact skeleton discovered in Ethiopia in the 1970s
Bipedal (known from pelvic structure)
Long arms, short legs, grasping feet
Brain size similar to chimpanzee
450 cc (Homo sapiens is 1,400 cc)
The Evolution of Human Beings
Homo ergaster
Ancestral to Homo erectus and Homo sapiens
Brain over half volume of Homo sapiens
Modern face, limbs, height, advanced tool technology
“Turkana boy”
Best-preserved remains of Homo ergaster
9 years old at death
1.6 million years old
The Evolution of Human Beings
Homo neanderthalensis
Lived 200,000 –27,000 years ago
First extinct hominin fossils found (1856)
Short, stocky, powerfully built
Heavy brow ridge, receding chin
Brain size slightly larger than Homo sapiens
Tool technology, burial of dead
Likely descended from Homo ergaster
Not ancestral to Homo sapiens
The Evolution of Human Beings
Modern Homo sapiens
Evolved modern anatomical form in Africa before migrating
Arose from 100,000 –200,000 years ago
Migrated to Indonesia, etc. by 46,000 years ago (coexisted with Homo erectus)
Arrived in Europe 40,000 years ago (coexisted with Homo neanderthalensis)
Homo sapiens replaced them –likely through competition
The Evolution of Human Beings
Homo floresiensis
“Hobbit people”
Discovered in 2004 on the Indonesian island of Flores
Lived as recently as 18,000 years ago
Three feet tall as adults
Cranial capacity of 380 cc
Bain size similar to “Lucy”
Used fire and sophisticated tools
The Evolution of Human Beings
Appeared in the literature 2006
Nothing like it before
Debate:
Diseased population
Preserved older
Normal variation
The Evolution of Human Beings
Homo floresiensis
This species raises some interesting questions
How did Homo floresiensis avoid extinction for so long?
How could they be so sophisticated in their tool and fire use with such small brains?
Did they evolve from Home erectus?