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

During the Archean Eon, prior to About 3.5 billion years ago, the floor of the seas would have looked like this.

Pre-biotic Earth

Barren rock and sediment

Stromatolite(structure constructed by cyanobacteria)

North Pole, Western AustraliaAge: Archean (3.5 billion years old)

By at least 3.5. billion years ago, Oldest life forms appeared: ProkaryotesCyanobacteria (photosynthetic bacteria) were among these early life forms

Filamentous Prokaryote Microfossils (Probably Cyanobacteria)

Apex Chert, Marble Bar, Western Australia

Age: Archean (3.46 billion years old)

Additional evidence of early life: prokaryote microfossils

By about 2 to 1.5 billion years ago (Proterozoic Eon) the first eukaryotes appeared (organisms will a well-defined nucleus and organelles).

These are distinguished in the fossil record by their larger size.

Acritarch(algal cyst)

From prokaryote to eukaryote

Symbiosis

cyanobacteria

plant cell

animalcell

purple bacteria

From prokaryote to eukaryote

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

Host prokaryote

Symbiosis

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

Oxygen production by cyanobacteria over several millions of years had, by this time, made enough oxygen to sustain

complex life forms in the sea

Latest Proterozoic (Ediacaran Period)

But then, something happened in the “Garden of Ediacara”…

Something wanted to eat…

Everything !

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 weaponsAnd 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

(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.

Oldest abundant skeletal remains

Earliest Cambrian (543 million years)

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 numberof phyla through time

View of Stephen Jay Gould:Sudden appearance of phyla,removal of many by later mass extinction

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 1

Oddball example 2

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.

Hallucigenia

(Phylum Onychophora…or something completely different ?)

From Middle Cambrian Burgess Shale, B.C.

Pikaiaearliest known Chordate

One of the Many Creatures That Appeared During the Cambrian Explosion

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

From the early chordates arose the first fishes(abundant by the Ordovician Period)

Jawless Fishes

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).

Include Coelocanths (Latimeria)

Lobefinned Fishes

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

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 Eusthenopteronhad evolved “amphibian-like” features, including fins that were strong enough to allow limited locomotion on land.

Eusthenopteron

Late Devonian: Lobefinned Fishes with Lungs

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

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)

Early True Mammals

Hadrocodium (Early Jurassic)Member of extinct group of early mammals

Monotremes(egg layers)

Multituberculates(extinct, rodent-like mammals)

Marsupials (pouched mammals)

Eutherians(placental mammals)

Divisions Within Class Mammalia

Mammals, in turn, diverged into 4 major subclassesWe belong to the eutherian (placental) subclass.

The Odd Primate

During the Late Tertiary early hominids appeared in the Africa savannas.

Within the placental mammals merged The Primates (our taxonomic order).

It would take about 5 million years of evolution to reach the current state of development of modern Homo sapiens.. 

Are We Not Men ? We Are Devo !

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