Download 10) The history of life on Earth has been marked by a number of

BIOL 1407 Instructor: Mr. Sanregret Lecture notes Chapter 25 (note: during lecture I presented points
1-9 as part of chapter 24, when they are actually in chapter 25. Sorry for the confusion.)
1) Macroevolution refers to the occurrence of major evolutionary innovations. These would include
new structures such an as air-breathing lungs, or new major taxonomic groups, like mammals, or
insects. Macroevolution is basically an extension of microevolution, where many small changes
over a long period of time result in large changes.
2) One objection to macroevolution is that complex structures like mammalian eyes cannot evolve in
one step, and a part of a complex structure is not functional. Thus, it is argued, since natural
selection only favors traits with immediate benefits, complex parts could not evolve by random
mutation and natural selection. However, this objection is flawed because it assumes that a
structure has the same function throughout its existence in the phylogeny (i.e., family tree of life).
E.g. a functional wing in the bat’s tree climbing ancestor could not appear in a one generation.
However, it is possible the there could be selective pressure for the digits and webbing of the
forepaws to lengthen, providing greater wind resistance to slow the rate of falling (either for
accidental falls, or for jumping from branch to branch). Over time a limb modified for air
resistance might allow for gliding. A limb modified for gliding may in time become adapted to
true flapping flight. See also the discussion in the textbook on the evolution of the human eye.
3) The evolution of biological novelties can in some ways be compared to the incremental
modification in human technology. Phone lines were originally put in for telephones, with no
clear expectation that in the future they would be used for the Internet and fax machines.
However, if phone lines had not been in place already, it is probable that those later technologies
would not have developed in the way they did.
4) Sometimes a structure that is used for one function but evolved within a different context is called
an “exaptation.” For example, using urine to mark territorial boundaries can be described as an
exaptation, since urination evolved as an excretory function before it was used for communication.
The need for this term is questionable, given that probably all adpatations are also exaptations.
5) Not all genes have the same magnitude of effect on an organism’s phenotype. Changes in genes
that have effects early in development generally result in major changes to the adult organism.
6) As an organism grows, their parts do not usually all grow at the same rate. This is called
allometry. This means that the organism’s shape changes from infancy to adulthood (e.g. human
infant vs. human adult). Differences in allometric relationships can result in major changes in
adult morphology. This is why related taxonomic groups usually look much more similar as
embryos than as adults.
7) Heterochrony means a difference in the rate or timing of the developmental events. E.g. starting
from a similar infant skull, the jaw of a chimpanzee elongates more rapidly than the jaw of a
human, resulting in a great difference in adult human and chimpanzee skulls. Note also that this
difference does not require many changes in genes.
8) Homeotic genes control fundamental features of body plan and structure. Different homoetic
genes determine the whether or not major body parts will develop, and the number and location of
body parts. For example, flies have had homoebox genes artificially altered resulting in legs
growing where they would normally have antennae. The expression of one type of homoebox
gene in the tips of vertebrate limb buds allows for the growth of digits.
9) One last note, evolution is not a chain of advancement but is instead a branching family tree. All
of the organisms that exist today are the most recent link in four billion year old lineage. Some
organisms, like bacteria and archaea, resemble their ancestors more than some other branches.
However, it is usually a mistake to think of one extant species as a more primitive version of
another extant species. Each has a unique history with unique adaptations.
10) The history of life on Earth has been marked by a number of “revolutions.”
11) The ~4.6 billion year history of the Earth is divided into three eons. The first is called the
Archaean, which lasted until 2.5 billion years ago.
12) The oldest microorganism fossils are 3.5 billion years old, so life originated on Earth no later than
that time. It is probable that prior to the existence of life, there was a primordial soup of organic
molecules. The first organisms probably subsisted on the primordial soup.
13) RNA was probably the basis for the first organisms. RNA can function as hereditary material
(e.g. in retroviruses). Like proteins, RNA molecules can take a variety of shapes and can have a
limited ability to catalyze metabolic reactions. These catalytic RNA molecules are called
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ribozymes. DNA only takes the shape of a double stranded helix and cannot catalyze reactions.
For this reason, the earliest life on Earth is theorized to have been an “RNA world.”
The processes that produced the primordial soup would not be rapid enough to sustain the growing
population of organisms. This provided selective pressure for metabolic pathways that allowed
organisms to make their own food (e.g., by photosynthesis).
All of the major metabolic pathways (e.g. the various forms of respiration and photosynthesis)
evolved during the Archaean. One of these metabolic pathways is a form of photosynthesis that
produces oxygen gas (O2) as a by-product.
Oxygen gas is a very reactive chemical, and easily reacts with (i.e. oxidizes) other chemicals in the
environment. For this reason, O2 would not have been present in the early atmosphere until a large
number of photosynthetic organisms were present producing it. In the earliest times, oxygen
would have been toxic to most organisms, due to its reactive nature. At some point, some
organisms adapted to the presence of oxygen by developing aerobic respiration.
The Proterozoic eon saw the development of the eukaryotic cell and multicellular organisms like
various algae (aquatic photosynthetic eukaryotes) and soft bodied animals.
There are two major ways in which eukaryotic cells evolved from prokaryotes: i. Infolding of the
prokaryotic plasma membrane. This produced the endomembrane system of endoplasmic
reticulum, Golgi complex, vesicles, and nuclear envelope. ii. Endosymbiosis: This occurs
when a smaller prokaryote is engulfed by a larger prokaryote. Over generations the two cells
adapt to each other and become dependent on each other.
Mitochondria and plastids almost certainly originated through endosymbiosis. Both have their
own loop of DNA (like a prokaryote). They also have binary fission, ribosomes, and size similar
to prokaryotes. Some of their essential functions require proteins coded by nuclear DNA, so
mitochondria and plastids cannot survive on their own outside their cell.
The third eon, which we live in now, is called the Phanerozoic and began with the Cambrian
explosion. The Cambrian explosion, 542 million years ago, is the sudden appearance in the fossil
record of animals with hard, easily-preserved shells and skeletons.
The Phanerozoic is divided into three eras. The first era, the Paleozoic, saw the colonization of
land by plants, amphibians, and insects. Forests of this era consist of ferns (or fern relatives). By
the end of the Paleozoic, all continents have moved together to form one supercontinent, Pangaea.
A massive extinction event called the Permian extinctions results in the loss of about 90% of the
existing species, and marks the boundary with the next era…
The Mesozoic era is known as the Age of Dinosaurs and followed the Permian extinctions,
beginning 251 million years ago. The reptiles and gymnosperms (e.g. modern day pine trees and
sago palms) underwent an adaptive radiation. They filled in the niches left by the extinct species
and dominated the Mesozoic terrestrial ecosystems. The Cretaceous extinctions left about 50%
of the existing species extinct and marked the end of the Mesozoic 65 million years ago.
The Cenozoic era is the Age of Mammals, and the era we live in now. Following the Cretaceous
extinctions, mammals and birds adaptively radiated to fill the niches left behind by the dinosaurs.
Most of the gymnosperm flora was replaced by angiosperms (flowering plants).
Note that from a botanical point of view, the Paleozoic is the Age of Ferns, the Mesozoic is the
Age of Gymnosperms, and the Cenozoic is the Age of Flowering Plants.