Bio-2
Handout Pages for the Chapter on Human Evolution


Over three and a half thousand million years ago (3.5 billion years ago) our planet was radically different in almost every way from the one we live on today. Enormous volcanoes, much more widespread than today, built up mountains of lava and ash. The atmosphere was filled with gasses such as ammonia, methane, hydrogen and steam. As the earth cooled, the water vapor condensed to liquid water, and rain began to fall in such enormous quantities that the oceans were produced. Now remember, this was happening over hundreds of millions of years. There was virtually no oxygen in the atmosphere. In consequence, there was no ozone layer and ultra-violet rays, in a strength that would be lethal to us, beat down onto the young planet.

Over millions of years the energy from volcanoes, meteorites, lightning strikes, ultra-violet rays, and even free radioactive discharges, allowed the simple gas molecules to produce organic molecules, such as nucleotides and amino acids. All these very small and simple molecules produced a chemical soup in the muddy waters. In 1953, Stanley Miller performed a now famous experiment. He placed a mixture of simple gases in a sealed tank, heated it, and added an electrical spark. All this was to simulate the early environment of earth. When Miller checked the contents of his tank after a while, he found a variety of different small organic molecules, such as nucleic acids, protein chains of linked amino acids, etc.

Since we already know that phospholipids will spontaneously form a lipid bilayer if placed into a water environment, once these very first phospholipids were formed in these small tide pools of water, they formed hollow 'cell membranes'. Over the course of millions of years, these hollow spheres began to collect and protect proteins and carbohydrates and nucleic acids. Eventually, these proteins were able to cause the nucleic acids to replicate, and even produce new protein. The first enzymes and self-replicating reactions just by chance happened. This collection of molecules that could make copies of itself, that could replicate was the first form of life on the planet.

Some of these bacteria-like organisms, like many which still survive today, produced oxygen as a waste product. They lived in the water and multiplied and evolved by adapting into all sorts of tiny, replicating creatures. Over the course of several million years they filled the oceans and with so much oxygen being released as their waste, they began to change the atmosphere from mostly nitrogen gas, carbon dioxide and other gases into the atmosphere we now breathe filled with lots of oxygen gas molecules.

An interesting and useful way of trying to understand the tremendous times for all of these events is to compare the existence of the earth to a regular 12 month calendar. If the formation of the earth was back in January, we now are in mid-June on our imaginary calendar.

Outwardly things changed little for hundreds of millions of years.

One major advancement for these self-replicating creatures to have an advantage over the other simple creatures was the ability to propel themselves through the water, and to be more energy efficient. It is believed that cilia and mitochondria are derived from a one celled organism combining with another, larger organism. A spirochete can wiggle and so move through water. If a primitive form of this cell were to combine with a larger cell, this new combined cell could have the remnant of the spirochete on its surface and so could propel itself through the water. If more were added, we now have a ciliated organism. The ciliates were very successful since they're covered with a coat of beating hairs - the cilia - which drives them through the water. The cilia also waft tiny particles of food into their gullets.

Now if a small cell were to be engulfed into a larger cell, and this small cell were used only for energy (ATP) production, that would be very energy efficient for this new combined cell. It is believed that is how mitochondria were derived. In fact, the mitochondrial membrane resembles a primitive bacterial cell membrane in many ways.

Any local pond can provide evidence of the next dramatic development. Around fourteen hundred million years ago (that's the middle of August on our calendar) some kinds of primitive cells began to collaborate to form multicellular creatures with some cells becoming more specialized for a particular function. Some of these multicellular creatures seem to have animal characteristics, while others appear to be simple plants, and even others seem to be half animal, half plant.

An example still around today would be a collection of cells, a mouth, and a gut. It is one of the comb jellies - which swim in the oceans but which are so transparent that they are hardly ever noticed. These are true animals with muscle fibers and a simple nervous system.

These tiny creatures will eventually produce their own sexual cells for release and fertilization in the sea. And then new colonies will appear. When sexual reproduction evolved, it greatly increased the potential for variation in offspring - and therefore the potential for new species. Around six to seven hundred million years ago (that's late October on our calendar) there was indeed a great increase in life forms in the sea; including jellyfish.

A even better adapted creature was the the flatworm. Propelled by tiny cilia on its surface, this develops into an animals that has a definite front end and back end, is sensitive to light, and can move in a purposeful way. A flat shape is not suited to burrowing in mud or sand. Nevertheless there is both food and safety at the bottom of the sea, and some worms changed from being flat to being round and long - a better shape for exploiting this habitat. Eventually worms developed protective shells over their backs and became mollusks. The first of them appeared five to six hundred million years ago (early November on our calendar). Some have a well developed head, with eyes and sensory tentacles, and a very efficient feeding organ - a long scraping tongue.

A few mollusks have gone to the other extreme and become free-swimming. Their shells have been reduced to mere scales over their bodies - or have been done away with completely.

In deeper waters swims a survivor of another successful group of mollusks which were very common in the seas of 500 million years ago - that's in November on our calendar - the nautilus. More common in the seas of today are their relatives, the squids. They have lost their shells. They use a kind of jet propulsion for fast swimming.

You may notice that we as humans are segmented (just consider the vertebral column) as were the primitive segmented worms. The segments, with their pairs of movable bristles, made sustained burrowing much easier. But some relatives have put them to more sophisticated uses. Crustacean are built upon that segmented plan, but each part has become specialized - as antennae. Or food manipulators, or legs, or for breathing or for reproduction.

All these creatures have an external skeleton and it is clearly a very effective way of building a body. But the jointed skeleton has a very special quality - mechanically, it works just as well on land as it does in the water. So animals with such characteristics were able to move out on to land without much difficulty. And that's exactly what happened around 400 million years ago - about mid-November in our calendar.

Each spring, on the eastern seaboard of North America, a strange re-enactment of that momentous episode in life's history takes place. Horseshoe crabs have changed little for several hundred million years and are relics of the huge variety of segmented creatures that once swam in the seas and among whom were the first invaders of land. Eventually similar segmented creatures successfully spent more and more time on land. From them evolved true land animals.

The land which, for so long, had been naked and sterile, had just begun to acquire patches of green algae, and lowly forms such as mosses and liverworts, were now beginning to grow at the edge of water.

Into these miniature jungles came other invaders from the sea - again animals with segmented bodies but in this case the ancestors of millipedes. They were vegetarians and the biggest today are only a few inches long; but many of the ancient forms that pioneered life on land grew much larger. One, indeed, was as long as a cow.

About four hundred million years ago, new creatures appeared which were to be forerunners of probably the most successful group of all animals without backbones - the insects. They fed on vegetable matter, but as plants grew taller, and food became more inaccessible, they had to do more climbing. And getting down again might sometimes have been worse than getting up. Maybe that was one reason for a dramatic development. Some little creatures developed wings for flying from plant to plant. Insects were the first creatures to take to the air - over 380 million years ago, and they were to have it to themselves for 100 million years.

By the time the first insects were flying, the plant life on earth had undergone dramatic changes. By developing new types of cell, plants had solved the problems of drying out and support out of water; and they had flourished and grown tall. But they still needed water to transport their sex cells from one individual to another - and that meant that they could still only grow in damp places. But eventually that problem too was solved by developing pollen and using the wind. It is a haphazard method of fertilization so pollen has to be produced in huge quantities. One cone may produce several million grains and there are thousands of cones on an average sized tree.

Plants now began to exploit the achievement of the insects in the air. They did so by evolving flowers - and color came to the world as never before. Flowers carry both male and female sex organs and they first appeared about 100 million years ago - about December 20th on our calendar. Their colors and scent are designed to attract insects, the nectar to reward them. The insects unwittingly carry pollen from the male part of one flower to the female parts of another, and plants have evolved an infinite variety of ways to increase the efficiency of this transport system. The sexual reproduction was as successful for the plants as it was for the animals.

With such abundant plant food on the land, there were more and more creatures venturing out of the water. They evolved into the first amphibians. And for about 100 million years -during the early part of December on our calendar - the land was dominated by giant amphibians. Few amphibians go far from water for their skins are not water-tight. If they dry out, they lose their body fluids and die. And each spring many amphibians must return to the water to lay their eggs - and there they seem more like their fish ancestors. The eyes of amphibians are similar to those of their fish ancestors, but they have developed an ability to blink and a membrane to keep them clean and moist. In air you do need quite a different hearing apparatus than you need in water - eardrums. And with them came a voice. Before amphibians had crawled out of the water 350 million years ago, the only animal sounds on earth were the whirrs and chirps of insects, so the first animal chorus to break the silence of the land may well have sounded like a chorus of primitive frogs.

From the amphibians, the reptiles evolved even better suited to live on the land with an egg with a waterproof shell. The other major innovation made by the reptiles concerns the nature of their skin. It's not moist, like a frog's, but tough, covered in scales - and most crucial of all, - practically water-tight. This skin has enabled reptiles to colonize the hottest and driest places on earth. In spite of their misleading reputation as cold-blooded creatures, reptiles can maintain a higher working temperature than ourselves. They don't generate it internally, but get it directly from the sun. In the morning, they can be found warming themselves up in the sun getting ready for the day's activities. Because they cannot sweat and cool themselves, overheating can be dangerous, so around mid-day they move down into the shade of the rocky clefts. By choosing resting places carefully they can keep their body temperature close to 37 degrees Celsius at all times. When they've reached their working temperature, they are able to go feeding. This economical method of obtaining heat means that reptiles can survive on about one tenth of the food that a mammal of similar size would require. And means they can live in places where food is very scarce, such as deserts.

The reptiles were the first backboned animals for which fertilization was essential, without water to allow sperm to swim to egg, there was no other method. The males had to place their sperm inside the female's body. Giant reptiles - the dinosaurs - dominated the earth, during mid-December on our calendar.

Some developed their scales into feathery insulators - and ultimately they became birds. But even when the large reptiles were ruling the world there were other little creatures that also had warm blood whose bodies were insulated not with feathers but with fir. They were the first mammals and they were to revolutionize the business of caring for the young. In time they gave rise to all the mammals that exist today, including ourselves.

They developed a new way to feed their newborn young. The tiny infants feed off of breast milk. The milk comes from a modified sweat gland - a mamma to give it its technical name, from which the word mammal is itself derived. They also developed a new technique of reproduction. They retain their babies inside them until they are well developed, nourishing them with a placenta. The amazing variety of mammalian forms are all derived from small nocturnal creatures that lived unobtrusively hidden in the trees while the giant reptiles ruled the earth.

Mammals arose from reptiles around 240 million years ago (mya). Dinosaurs (reptiles) were the dominant creatures on the planet at the time. The dinosaurs had adapted (evolved) to occupy every available niche. The mammals could not compete very successfully with the dinosaurs and so were small, nocturnal, and tree-living. The mammals were a very minor population of organisms for 175 million years. About 65 mya the dinosaurs became extinct over several million years. The mammals then filled in all the newly available habitats or niches. At the same time the plants were diversifying which provided the growing numbers of mammals ample food and more places to live. About 40 mya, from the mammals, the first primates appeared. They lived in trees.

They possessed:
four fingers and an opposable thumb for climbing
nails (not claws)
freely moveable shoulders and hips
eyes in the front of their heads (3-D vision)
larger brains (sharper vision, increased coordination)
complex social behaviors

Two groups of Primates:
(1)Prosimians (before apes: lemurs, lorises)

(2)Anthropoids (monkeys, apes, humans)

The first primates were the prosimians, about 40 mya. They lived in trees and were nocturnal. About 38 mya, a population of prosimians arose into the anthropoids in Africa (and possibly Asia) and spread to Europe. These were monkeys that were now active during the day with larger brains.

Old World Monkeys: Asia and Africa

New World Monkeys: Central and South America

The old world monkeys diversified into "Homonoids" (ancestors to humans) and into apes. These creatures had no tails, walked on their knuckles and lived on the ground more of the time, less time in the trees. By comparing DNA sequences, Homonoids (and humans) are more similar to chimpanzees than they are to apes. The homonoids walked upright and so had a more human like curved spine, shorter pelvis for walking muscles, foramen magnum (the opening at the base of the skull that the spinal cord exits the skull through) is toward the bottom of the skull (as in humans) and less toward the back of the skull (as in apes), big toe in line with the other four toes (less climbing, better walking), rounder jaw (ape's jaw is V-shaped), flatter face.

About 4.4 mya was the first homonid (precursor to humans). These were of the genus "Australopithecus": (Austral=southern) + (pithecus=ape)

Several Species:
Australopithecus Ramidus (4.4 mya)
A. Afarensis (Lucy) (3-4 mya)
A. Africanus (3.0 mya)
Homo habilis (Homo: talk, use and make tools)
Homo erectus (larger brain, used fire)
Homo sapies (200,000 years ago, Neanderthal man)


5 Kingdoms
All living creatures can be placed into one of five groups or kingdoms.
These five kingdoms are:
Monerans
Protists
Fungi
Plants
Animals


Charles Darwin: Radical Ideas of:
1)Evolution: genetically based adaptations
2)Natural Selection: modification by descent, forced by the environment


Human Evolution
After grouping all living things into the five kingdoms, within each kingdom you can subgroup organisms according to their similarities (skeletal, protein, DNA, behavior, reproduction, and so forth). These subgroups are called "Taxons", and they become more and more specific. The taxons are:
Kingdom
Phylum
Class
Order
Family
Genus
Species

Human fit in this way:
Kingdom=Animalia
Phylum=Chordata (spinal cord)
Subphylum=Vertebrata (boney vertebral column)
Class=Mammalia (hair, feed young with milk)
Order=Primate (opposable thumb, frontally directed eyes)
Family=Homindae (walk upright, no tail, long legs, big brains)
Genus=Homo
Species=sapiens


Embryo: Weeks 1-8 of development.         Fetus: Weeks 9 - end of pregnancy.

The human embryo during its development has gills, a tail, webbing between the toes and fingers, and spends its entire time floating and developing in the amnionic fluid (which has a similar salt concentration as ocean water).

Vestigial structures: Useless structures left over from a common ancestor. A good example would be the hip bones in a whale. Obviously a California gray whale does not walk, but since it is a mammal, and related to us humans, it does still in its skeleton have small remnants of pelvic bones. You and I have muscles attached to our ears. We don't use them to rotate our ears, but our four legged mammalian early relatives did.



Homologous structures

Homologous structures: When creatures branch off into different species and into different evolutionary directions, they retain evidence of their shared body structures. You can see from the diagram that reptiles, chickens, bats, penguins, and even you and I share a common structure to our arm bones.

Analogous structures: Sometimes unrelated organisms will have similar structures. A very good example would be the wing of a bat and the wing of an insect. They both evolved to allow for flight. But the insect wing has no bones and is not related at all to the wing of a bat.


Textbook Review Topics for Final Exam

That's it! Good luck!