Download Let us now take a look at how life began and evolved into the

1/12/17
Let us now take a look at how life began and evolved into the species we see today. Evidence
indicates that all living things descended from a single common ancestor through a branching of
lineages. The important thing to remember here is that life started as relatively simple cells in an
aquatic environment and evolved into more intricately organized forms; existing traits were
modified in response to an organisms changing environment.
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The earth formed about 4.5 billion years ago; there was no water and the atmosphere was noxious.
The planet began to cool and water formed the oceans. These warm, salty seas created more stable
conditions for life to evolve.
The current concept is that this very early Earth had what we call a prebiotic soup; it had all kinds of
chemicals, some of which were components of the building blocks of life. All living things reproduce,
copying their genetic material and passing it on to their offspring. Thus, the ability to copy the
molecules that encode genetic information is a key step in the origin of life — without it, life could not
exist. The formation of self replicating RNA may have evolved and been moving around in these
chemical soups. Some scientists call this the “naked RNA planet”: a time in early earth history when
RNA was not contained within a cell and was free-flowing. Eventually we moved to a point where
these molecules evolved into the first cellular life forms, when cell walls formed and these RNA
molecules were captured within. At this point we then we had the genesis of the three domains of life,
Eukarya, Archaea, and Bacteria. This three domain classification systems was introduced by Carl
Woese in 1977. Woese’s proposed this segmentation according to ribosomal RNA (rRNA).
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How life originated and how the first cell came into being are matters of speculation; between 4.0
and 3.7 billion years ago, cell walls formed and life appeared. The three Domains of life, Bacteria,
Archaea and Eukarya evolved from a common ancestor and were probably anaerobic.
Bacteria are single-celled microorganisms that lack a nucleus inside the cell wall (prokaryote).
Archaea are also prokaryotes and lack a nucleus but are more closely related to Eukarya than to
Bacteria. Archaea are able to adapt quickly to extreme environments, such as high temperatures,
high acids, high sulfur, etc. Archaea are able to produce methane instead of carbon dioxide in
metabolism.
Eukarya are organisms that contains a membrane bound nucleus (eukaryote). They are often multicellular organisms but some may be simple cell eukarya that are microscopic
It was not until approximately 3.0 billion years ago that photosynthesis evolved and oxygen was
released into the water and eventually accumulated in the atmosphere.
Life remained mostly small and microscopic until about 580 million years ago, when complex
multicellular life arose from the eukaryotes. This includes the animals and plants.
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Life was bound to water for the first 3 billion years; these animals were invertebrates. Invertebrates are
any animal lacking a backbone, including all species not classified as vertebrates. Most animals today are
invertebrates such as corals, insects, worms, jellyfish, starfish, and snails. The first shells appearing in the
fossil records denote the start of the invertebrate group arthropods, which we will be discussing as a group
later.
The jawless fish were the first vertebrates to appear. Vertebrates are any chordate, characterized by a bony
or cartilaginous skeleton and a well-developed brain. The group contains the five vertebrate classes: fish,
amphibians, reptiles, birds, and mammals.
The Sculpture Learning Plaza has examples of aquatic single-celled organisms including amoeba,
vorticella and stentor and plaques of many more. San Francisco Zoo
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Fish were the first class of animals to develop a backbone; they include the bony fish (i.e. carp,
tuna) and the cartilaginous fish (sharks, rays). We will not be going into detail of the fish class as
they are not part of the primary exhibits of the zoo. We do have some carp in the children’s zoo
that like to hide under the bridge.
Ectothermic means an animal adjusts their internal body temperatures through behaviors that
depend on temperature differences within their environment; they depend on external thermal
sources such as sunlight or a heated rock surface.
In addition, fish generally are covered with scales; their limbs are modified into fins for
swimming; they breathe with gills; and lay eggs in the water.
Fish as a whole have spread into every available kind of environment, however, each species has
tolerance for only a limited range of environmental conditions.
The Sculpture Learning Plaza has examples of fish including the freshwater eel, hammerhead
shark, piranha, mudskipper and a coelacanth.
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Plants were the first multicellular organisms to invade land. Sunlight was a limiting factor in the ocean
and plants began spreading out on the surface to get more sunlight. With the invasion of plants onto the
landscape, the available oxygen in the environment rose, making it possible for aerobic organisms to exist
on land.
The evolutionary transition from water to land occurred over millions of years. In transitioning to land, an
organism had to withstand the effects of gravity, had to be able to breath air, had to minimize water loss
and be able to regulate their temperature and also to adjust their senses so they are suited for air instead of
water.
The first animals to live on land were arthropods, probably ancient centipedes and scorpions. Land was
an unoccupied habitat; oxygen was accumulating in the atmosphere and their was now a potential food
source available. The arthropods of the ocean were great candidates to come onto the land because they
already had an exoskeleton, a hard shell to help maintain water balance. When flowering plants evolved
in the Cretaceous period, insects thrived.
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The first limbed vertebrates evolved from the lobe-finned fish about 360 million years ago. They
probably only occasionally ventured on land at first, from freshwater bodies. They crawled out of the
shallow seas and swallowed gulps of air with primitive lungs, found less competition for food on
land and were able to avoid large predatory fish.
Once tetrapods or four-legged vertebrates developed digits on their limbs, they were able to adapt
more to walking on land than paddling in the water. The insects had already invaded land which
provided a food source for vertebrates to exploit as they moved onto dry land.
The Ichthyostega that you see here was a transitional fossil between fish and tetrapods, combining a
fish-like tail and gills with an amphibian skull and limbs. Another transitional fossil species,
Tiktaalik roseae, was found in 2004 and is a transition between earlier lobe-finned fish and
amphibians; it tells us a lot about how vertebrates came to live on land. San Francisco Zoo
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Amphibians first appeared in the fossil record about 380 million years ago, evolving from a tetrapod
ancestor. They were the first vertebrates to live on land; the transition from aquatic to terrestrial living is
indicated in the Class Amphibia.
Amphibians are ectothermic animals who spend part of their time on land and part in the water. Despite
their adaptations to terrestrial life, they still require cool, moist environments for their jelly-like eggs and
their adult gas-permeable skin. Their skin must remain moist to aid in breathing. This permeable skin
makes amphibians particularly vulnerable to environmental disturbances, from chemical pollution to the
thinning ozone layer and global climate change.
Amphibian means to lead a “double life”. Amphibians can live or function on land and in water.
Amphibians are the only land vertebrate that go through a change in the larval, or tadpole, state into
adult. This is known as metamorphosis. The tadpole lives in a niche that is different from the adult’s,
resulting in less competition within the species; metamorphosis reduces competition between the tadpole
and adult and allows for specialization for feeding and reproduction.
In time, amphibians moved out of the water to take advantage of the burgeoning amount of prey on land.
What kept them tied to the water was the need to lay their eggs there and most still had a water-bound
larval form. Amphibians were the only vertebrate life forms on land for about 60 million years. They
became rare and diminutive as better-adapted reptiles began to take over the land.
Amphibians have a three-chambered heart and lacked claws on their toes; the front foot had four toes.
They have external fertilization. The living representatives of this class include frogs, toads, newts,
salamanders and limbless caecilians.
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The development of the amniotic egg in the late Carboniferous period saw the rise of the reptile
class and was the most significant adaptation leading to full time land habitation. Reptiles first
arose from amphibians in the swamps present during this time period.
As feeding shifted from an aquatic environment to a terrestrial one, animals needed more
powerful jaw muscles for getting through tough arthropod exoskeletons and the courser plant
material. The early diversification of amniotes produced holes in the temporal region of the skull,
provided space for the large muscles needed for feeding. These fenestrae allowed for the
attachment of larger, stronger jaw muscles resulting in more powerful and faster bites.
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The tetrapods like reptiles, birds and mammals are referred to as amniotes. The amniotes have
certain membranes associated with embryos inside the egg. It is an adaptation in terrestrial forms
during development.
With the amniotic egg, animals were no longer restricted to having to be near water to lay eggs.
The shell is semipermeable and keeps the embryo inside an internal membrane from drying out
but still allows the exchange of gases. Metamorphosis was no longer needed as the bridge to a
land form. The yolk inside the egg fed the embryo until hatching.
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Reptiles evolved from amphibians and are differentiated from amphibians by a stronger, more
effective jaw mechanisms, more effective locomotion on land, the amniotic egg and scales.
Like amphibians and fish, reptiles are ectothermic vertebrates. Being on land however they had
water-tight skin for water conservation and overlapping scales to protect them from the harsher
terrestrial environment with its sand and rocks. Ribs are found on most reptilian vertebrae to
protect their internal organs. One of the main differences between reptiles and amphibians is seen
in the jaws, as reptiles developed a greater biting strength than their amphibian ancestors.
Reptiles have three-chambered hearts (except for alligators and crocodiles, which have fourchambered hearts). The three-chambered heart can adjust the proportion of blood that goes to the
body versus the lungs. Reptiles use this ability to shift blood between the body and the lungs to
accelerate heating and slow cooling.
The word "reptile" comes from Latin, meaning “to crawl”. Reptiles have claws on their toes, with
the exception of the legless species, such as legless lizards and snakes. They have five toes on
both their front and rear feet.
Lastly, we have already mentioned the amniotic egg that reptiles lay; fertilization is internal.
Reptiles are represented by lizards, snakes, turtles, tortoises, alligators, crocodiles and the tuatara
lizard.
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At the end of the Paleozoic Era, when reptiles were beginning to radiate across the lands, there was a mass
extinction event. Over 90% of marine life and 70% of land species went extinct. This open up new areas for
species such as the dinosaur to radiate into. The Mesozoic Era became the “Age of Dinosaurs”.
Dinosaur means “terrible lizard”. They were different from the reptiles in that they had an erect stance; an
erect stance positions the limb bones directly under the body. This allows the limb bones to passively support
the body’s weight without muscles having to strain. This was in contrast to the reptile’s sprawling stance who
held its legs out to either side of its body. For example holding yourself in a push up position requires a lot of
effort just to keep yourself in that position. However, if you straighten out your arms and hold them directly
underneath you in an upright manner, it becomes much easier. Creatures with an erect stance use less energy
to move around and using less energy can be a big evolutionary advantage.
Modern birds are derived from a bipedal dinosaur group, the theropods and are the only living descendants of
the dinosaurs.
There was another mass extinction at the end of the Mesozoic Era due most likely to a meteorite. All
dinosaurs except birds went extinct at this time. This opened up new niches for the mammals who went
through adaptive radiation, allowing the next era, the Cenozoic, to be the “Age of Mammals”.
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The first true bird, Archaeopteryx, evolved during the late Jurassic Period.
Thomas Henry Huxley was a close colleague of Charles Darwin and one of the earliest advocates
for the theory of evolution. Huxley was also the first scientist to recognize that birds evolved from
dinosaurs, and he cited the newly discovered specimens of Archaeopteryx as fossils of a “missing
link” between dinosaurs and birds. Archeopteryx was discovered in 1860 in limestone deposits in
Germany. Archaeopteryx possessed characteristics of both birds and reptiles and was viewed as
the perfect example of an intermediate species that bridged the gap between two major classes of
vertebrate groups. It still retained many reptilian features such as claws on the front of the wings,
sharp teeth, and a long bony tail.
Archaeopteryx might have been identified as a theropod dinosaur but this specimen possessed the
most important avian characteristic, feathers. No dinosaur specimens at this time were known to
possess feathers. Archaeopteryx probably walked and fluttered around more than high flying as its
tail would not allow much lift, but its feathers were assymetrical, which are needed for flight. This
species had pneumatized bones, and a reversed hallux toe bone like all modern birds.
Since Archaeopteryx, newly discovered fossils showed that birds were not the only animals to
develop feathers; several dinosaur groups had feathered species as well.
Archaeopteryx was bipedal, feathered, had a furcula or “wishbone” and was possibly
endothermic. The furcula is formed by the fusion of the two clavicles; it functions in
strengthening the thoracic skeleton to withstand the rigors of flight.
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Birds belong to a specialized branch of the dinosaur family tree; all non-avian dinosaurs went
extinct at the end of the Cretaceous period.
Most major characteristics of birds can be directly related to their adaptations for flight.
Birds are endothermic vertebrates and were able to regulate their own body temperatures through
metabolic processes. More than any other trait, the ability to maintain a constant, high body
temperature has allowed birds and mammals to live in all possible habitats and at all seasons.
Endothermy required an increase in food intact to be able to generate enough heat to maintain a
constant internal body temperature. Bird’s skin is covered with feathers, which were modified
scales; birds retained the scales on their legs.
Birds have four-chambered hearts; their bones are lightweight and usually pneumatized with
criss-crossing struts for structural strength. Their forelimbs are modified as wings and they
lay eggs. They also have an efficient unidirectional respiratory system. Unidirectional flow
means that air moving through bird lungs is largely 'fresh' air & has a higher oxygen content. In
contrast, air flow is 'bidirectional' in mammals, moving back and forth into and out of the lungs.
As a result, air coming into a mammal's lungs is mixed with 'old' air (air that has been in the
lungs for a while) & this 'mixed air' has less oxygen. So, in bird lungs, more oxygen is available
to diffuse into the blood.
Reduced tail bones and distinctive feet are what set birds apart from dinosaurs.
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It was originally thought that feathers evolved for flight and were unique to birds. The fossil
record show that there were dinosaurs also with feathers. What are some of the possible uses for
feathers, other than for flying?
Could it be camouflage, sexual display, thermal insulation, or all of the above?
D is the correct answer. All of these are potential uses for feathers. We now know that feathers
evolved well before flight in dinosaurs. And it is likely that there were dinosaurs that used
feathers for camouflage, sexual display and thermal regulation. Feathers are also used for
waterproofing.
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Mammal ancestors were mammal-like reptiles that were around about the same time as the
emergence of dinosaurs. The earliest identifiable mammals were tiny, shrew-like mammals and
emerged during the late Triassic period about 200 million years ago. Early mammals were
probably nocturnal in order to avoid competition with the dominant carnivorous dinosaurs during
this time period. They were probably mostly insectivorous and lived in the trees. Early
mammal’s superior sense of smell and hearing, backed up by a larger brain, facilitated entry into
nocturnal niches with less exposure to dinosaur predation. The nocturnal lifestyle may have
contributed greatly to the development of mammalian traits such as endothermy and hair.
Mammal relatives are the synapsid reptiles having a single temporal fenestrae behind each orbit.
The fenestrae may have provided new attachment sites for jaw muscles. Early mammals were
endothermic, had fused lower jaws, three ear bones, and milk glands.
The three divergent lines of mammals based on their reproductive strategy all evolved during the
Cretaceous period; these being the monotremes, marsupials and placental mammals.
Monotremes are the oldest living group of mammals, with only two surviving groups, the
platypus and echidnas, which lay eggs.
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The Mesozoic era is considered the “Age of the Dinosaurs”. When the dinosaurs died off 65.5 million
years ago and the Cenozoic era, the “Age of the Mammals”, began, mammals were able to radiate into
all the niches that were left vacant by the dinosaurs and marine reptiles. Part of the successful strategy
was the differentiated teeth (heterodonts) that allowed them to adapt to a variety of diets and
environments. Fish, amphibians and most reptiles were homodonts and their teeth were all similar.
Mammals are endothermic vertebrates. They have hair at some point during their life cycle, they
breathe air, have live births, with the exception of the monotremes and they produce milk from their
mammary glands to feed their young.
A mnemonic for mammal characteristics is WHALE: Warm blooded, Hair or fur, Air to breath, Live
birth, Eats mother’s milk.
We will discuss the differences in reproductive strategies of monotremes, marsupials and true placental
mammals when we talk about the mammalian class.
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The geologic time scale is used to describe the timing and relationships between events that have
occurred throughout Earth’s history. It is a good tool for understanding the history of Earth and the
evolution of life.
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Definitions:
Prokaryote: an organism that lacks a nucleus
Eukaryote: an organism that contains a membrane bound nucleus
Tetrapod: a four-legged vertebrate; this group includes amphibians, reptiles and mammals.
Ectothermic: any animal whose regulation of body temperature depends on external sources, such as sunlight or a heated
rock surface
Endothermic: any animal dependent on or capable of the internal generation of heat. The animal is able to maintain a
relatively constant internal temperature, irrespective of the temperature of the surroundings.
Invertebrate: any animal lacking a backbone, including all species not classified as vertebrates. Most animals are
invertebrates. Corals, insects, worms, jellyfish, starfish, and snails are invertebrates.
Vertebrate: any chordate animal of the subphylum Vertebrata, characterized by a bony or cartilaginous skeleton and a
well-developed brain: the group contains fishes, amphibians, reptiles, birds, and mammals.
Chordate: a phylum comprising animals having at least at some stage of development a more or less well-developed
notochord, a dorsally situated central nervous system, and gill clefts in the walls of the pharynx and including the
vertebrates, lancelets, and tunicates. In most vertebrates, the notochord is replaced by vertebrae, a backbone.
Notochord: a primitive backbone; a flexible rodlike structure that forms the main support of the body in all chordates
during some stage of their development. In vertebrates, the notochord develops into a true backbone in the embryonic
phase. Primitive chordates, such as lancelets and tunicates, retain a notochord throughout their lives.
Metamorphosis: a major change in the form or structure of some animals or insects that happens as the animal or insect
becomes an adult
Anapsid: the skull does not have openings near the temples; no temporal fenestrae. Examples are the turtles and tortoises.
Synapsid: having one opening in the skull behind each eye; one temporal fenestrae. Examples are the mammals
Diapsid: having two openings in the skull behind each eye; two temporal fenestrae. Examples are major reptiles, the
dinosaurs and all birds.
Adaptive radiation: process in which organisms diversify rapidly into a multitude of new forms, particularly when a
change in the environment makes new resources available, creates new challenges and opens environmental niches.
Homodonts: animals which have similar types of teeth.
Heterodonts: animals which have different kinds of teeth; for example, most mammal teeth are differentiated into incisors,
canines and molars.
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