Where did we come from ?:
A whirlwind tour of evolutionary trends

Pre-biotic Earth
During the Archean Eon, prior to About 3.5 billion years ago, the floor of the seas would have looked like this.
Barren rock and sediment

By at least 3.5. billion years ago, Oldest life forms appeared: Prokaryotes
Cyanobacteria (photosynthetic bacteria) were among these early life forms
Stromatolite
(structure constructed by cyanobacteria)
North Pole, Western Australia
Age: Archean (3.5 billion years old)

Additional evidence of early life: prokaryote microfossils
Filamentous Prokaryote Microfossils
(Probably Cyanobacteria)
Apex Chert, Marble Bar, Western Australia

From prokaryote to eukaryote
Acritarch
(algal cyst)
By about 2 to 1.5 billion years ago (Proterozoic Eon) the first eukaryotes appeared (organisms will a welldefined nucleus and organelles).
These are distinguished in the fossil record by their larger size.

From prokaryote to eukaryote cyanobacteria
Host prokaryote
Symbiosis purple bacteria
Symbiosis plant cell animal cell
Organelles of eukaryotes (mitochondria in animals,and both mitochondria and chloroplasts in plants) are believed to have been produced via the introduction of prokaryotes into larger “host” prokaryotes and the establishment of a symbiotic relationship between the host cell and the smaller prokaryotic cells that they engulfed.
Purple bacteria became mitochondria, cyanobacteria became chloroplasts

First Experiment in Multicellularity
By about 600 to 575 million years ago (latest Proterozoic), the first multicellular animals appeared.
All of these were soft-bodied, and are known only from impressions

Latest Proterozoic (Ediacaran Period)
Oxygen production by cyanobacteria over several millions of years had, by this time, made enough oxygen to sustain complex life forms in the sea

But then, something happened in the “Garden of
Ediacara”…
Something wanted to eat…

Early Cambrian Period (Paleozoic Era, Phanerozoic Eon)
Between about 543 and 510 million years ago, skeletonized organisms appeared in a huge explosion of diversity.
This event is called The Cambrian Explosion.
Seascapes changed…
From peaceful oasis…
…to war zone with weapons and armour

A Short Note on Taxonomy
Before we proceed with the rest of this lecture, we should consider how organisms are classified.
In essence, our standard method of classification of life forms in the animal kingdom considers five basic hierarchical divisions:
Phylum (e.g. Chordata)
Class (e.g. Mammalia)
Order (e.g. Primates)
Family (e.g. Hominidae)
Genus (e.g. Homo )
Species (e.g. sapiens )

Cambrian Explosion
Nearly all of the phyla (highest level of classification for multicellular organisms) with skeletons that are present on Earth today originated in the Cambrian Period.
Why ?
The world’s first arms race
Organisms that developed hard mouthparts preyed on other animals.
In turn, prey had to develop some means of protection (hard skeletons or greater mobility)
Predators improved their weapons
And prey had to further improve their defenses…
Etc., etc.

Results of the Cambrian Arms Race
Different groups of organisms developed their weapons or defense systems in their own way, resulting in a wide array of body plans suited to various ways of life.
Ediacaran Period Cambrian Period

Oldest abundant skeletal remains
Earliest Cambrian (543 million years)
(scale bar: 1 mm)
Hydroxyapatite (phosphate-bearing mineral) was widely used for skeletal construction, even among the earliest skeletonized organisms.
Why use such a scarce material to make skeletons ?
We will answer this next day.

Significance of Oddballs
In addition to the basic animal body plans that appeared in the Cambrian and survived to the present day, a number of oddball forms also appeared.
Paleontologist Stephen Jay Gould boldly suggested some Cambrian phyla were represented by few or single species (the oddball forms)
If so, it is possible that the Cambrian Explosion produced more phyla than are present today extinction
Conventional view:
Gradual increase in number of phyla through time
View of Stephen Jay Gould:
Sudden appearance of phyla, removal of many by later mass extinction

Oddball example 1
Opabinia
(Phylum “Lobopoda”)
Combined characteristics of arthropods and worms
(does not fit into a present-day phylum)
From Middle Cambrian Burgess Shale, B.C.

Oddball example 2
Hallucigenia
(Phylum Onychophora…or something completely different ?)
From Middle Cambrian Burgess
Shale, B.C.
Original reconstruction Current reconstruction
Hallucigenia is sufficiently weird that people have argued which end was the head and which end was the tail, and which side was the bottom and which side was the top.

One of the Many Creatures That Appeared During the Cambrian Explosion
Pikaia earliest known Chordate
A notochord (an internal band of elastic tissue that could be flexed by muscle packs down its length) allowed early chordates to swim without the burden of heavy external armour.
We ultimately evolved from an animal that looked like this !

An Aside: Preservation of Burgess Shale Fauna
Burgess Shale organisms living on foot of escarpment (and possibly on edge and top of escarpment as well) smothered by due to slumping, killed and buried instantly
Rapid burial + low oxygen (+ possible mineralization during early decay)
= exceptional preservation

Jawless Fishes
From the early chordates arose the first fishes
(abundant by the Ordovician Period)
These fishes were covered with bony armour and lacked jaws

Jawed Fishes
From the jawless fishes arose the jawed fishes (abundant by at least the Silurian Period).
Jaws are thought to have developed via the modification of one pair of gill arches (structures that support gills slits).

Lobefinned Fishes
Include Coelocanths ( Latimeria )
One group of jawed fishes was the lobefinned fishes.
The Coelocanth is one of the only lobefinned fishes that has survived to the present day.
The uniquely structured fins served as precursors to legs

Late Devonian: Lobefinned Fishes with Lungs
Some lobefinned fishes developed lungs.
One group of these lung-bearing lobefinned fishes became the ancestral group to the earliest amphibians.
Lobe-finned fishes such as Eusthenopteron had evolved “amphibian-like” features, including fins that were strong enough to allow limited locomotion on land.
Eusthenopteron

Fish to Amphibian: Skeletal Modifications
Amphibians had to strengthen their shoulder and pelvic girdles to support their body weight
(did not have the luxury of water support).
Tongue cartilage was transformed into the first earbone (stapes).

Amphibian to Reptile
To make the final transition to land, a amphibians had to:
1.
Free themselves from a dependence on water for reproduction (this was solved via the development of the hard-shelled amniote egg).
2.
Develop a water-tight body covering to prevent drying out (solved through the development of thicker skin and scales).

Family Tree of Reptiles
From an ancestral reptile arose several subgroups, including the archosaurs (including turtles, crocodiles, dinosaurs, birds), flying reptiles (pterosaurs), and marine reptiles.
A group of special significance to us is the mammal-like reptiles
(therapsids) that appeared by the latest
Paleozoic.
No, you’re not expected to know all these names !

Reptiles to Mammals: hearing with our jawbones
Evolution of Ear: stapes acquires “stirrup” shape articular bone in jaw becomes malleus quadrate bone in jaw becomes incus

Early True Mammals
By a stroke of luck, one group of therapsids gave rise to mammals
(Class: Mammalia) during the Triassic
(The first true mammals appeared on Earth together with the earliest dinosaurs during the Triassic)
Hadrocodium (Early Jurassic)
Member of extinct group of early mammals

Divisions Within Class Mammalia
Multituberculates
(extinct, rodent-like mammals)
Marsupials
(pouched mammals)
Monotremes
(egg layers)
Eutherians
(placental mammals)
Mammals, in turn, diverged into 4 major subclasses
We belong to the eutherian (placental) subclass.

The Odd Primate
Within the placental mammals merged The Primates
(our taxonomic order).
During the Late Tertiary early hominids appeared in the
Africa savannas.

Are We Not Men ? We Are Devo !
It would take about 5 million years of evolution to reach the current state of development of modern Homo sapiens ..

Why Consider Evolution in Medical Geology ?
We are the product of at least 3.5 billion years of evolution.
The functioning of the human body embodies all the traits we have acquired during the course of our evolution, how we have interacted with other life forms during our recent evolution, and the changes in our environment as our circumstances change with the progress of technological development.
The next lecture will deal with examples of how these factors influence the current state of our health.

END OF LECTURE