chapter 26 biology

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Earth changed over geological time

So much change has occurred since the Earth formed that no rocks exist from the first 500 to 700 million years of Earth’s history (called the Hadean eon) that preceded the earliest fossils. Although it is impossible to be certain what early Earth was like, geological evidence is consistent with a meteor hitting the Earth almost 4.6 bya with such force that that debris from the impact formed the Moon. The rocky mantle of the Earth literally melted as atmospheric temperatures exceeded 2000°C.

Hadean Earth was also pummeled by asteroids, which could potentially vaporize entire oceans. Shifting between a fiery and a sometimes frozen Earth, it was a wildly dynamic environment that was unlikely to have supported life.


CO2 levels shifted, affecting temperature

The early atmosphere likely had high levels of CO2, and water slowly vaporized from the molten rock. The Earth cooled over a 2-million-year period. As the temperature cooled, clouds made of silicate condensed in the atmosphere and rained down, forming a warm ocean under a CO2 atmosphere. CO2 levels dropped and the Earth cooled, and for a period of time the ocean froze.


Continents moved over geological time

Earth’s crust formed rigid slabs of rock called plates under both continents and oceans. These huge slabs shift a few centimeters each year, a process called plate tectonics. The term tectonics comes from the Greek word for build or builder, and plate movements built and continue to build the geological features of Earth.


Origins of Life

Because we cannot recreate the process now, we have to use various lines of scientific exploration to piece together the puzzle of life’s origins, beginning with the geology of early Earth. Hadean Earth was a hot mass of molten rock about 4.6 bya. As it cooled, much of the water vapor present in Earth’s atmosphere condensed into liquid water that accumulated on the surface in chemically rich oceans. One scenario for the origin of life is that it originated in this dilute, hot, smelly soup of ammonia, formaldehyde, formic acid, cyanide, methane, hydrogen sulfide, and organic hydrocarbons. Whether at the oceans’ edges, in hydrothermal deep-sea vents, or elsewhere, life likely arose spontaneously from these early waters.


Early organic molecules may have originated in various ways

Organic molecules are the basis of all living organisms. How the first organic molecules formed is not known, and some could have extraterrestrial origins. Hundreds of thousands of meteorites and comets are known to have slammed into the early Earth, and recent findings suggest that at least some may have carried organic materials.


Organic molecules may have originated on early Earth

To carry out their experiment, Miller and Urey (1) assembled a reducing atmosphere rich in hydrogen and excluding gaseous oxygen; (2) placed this atmosphere over liquid water; (3) maintained this mixture at a temperature somewhat below 100°C; and (4) simulated lightning by bombarding it with energy in the form of sparks.


Metabolic pathways may have emerged in various ways

Many hypotheses for the emergence of metabolic pathways exist. One scenario assumes that primitive organisms were autotrophic, building all the complex organic molecules they required from simple inorganic compounds, rather than heterotrophic and acquiring all organic compounds from the surrounding environment. For example, glucose may have been synthesized from formaldehyde, CH2O, in the alkaline conditions that could have existed on early Earth. Glycolysis and a version of the Krebs cycle (see chapter 7) that functioned without enzymes are proposed to be the core from which other metabolic pathways emerged. Early autotrophs could have made, stored, and later used glucose as an energy source.

According to the hypothesis of an RNA world, RNA, rather than DNA, was the first nucleic acid that permitted self-replication, an important step toward life. Later DNA, which is more stable than RNA, took over the information storage function. Proteins that have a greater variety of building blocks (amino acids) gained the enzymatic function.


Ribozymes are RNA sequences with an enzymatic function.

Strong evidence supporting the RNA world hypothesis comes from the ribosome, which is used in cells for translation of RNA into proteins. Although the ribosome is composed of both protein and RNA, it is an RNA sequence that is involved in the central mechanism for translation. This is consistent with the hypothesis that early cells used RNA to catalyze the synthesis of peptides from an RNA sequence.


Single cells were the first life-forms

At some point, these bubbles became living cells with cell membranes and all the properties of life described in the chapter introduction. For most of the history of life on Earth, these single-celled organisms were the only life-forms. We don’t know exactly how cells formed because we can’t recreate that process, but at some point simple cellular life evolved.


Fossil evidence indicates life may have originated 3.2 bya

Nonbiological processes can produce microfossil-like structures, and rocks older than 3 billion years are rarely unchanged by geological action over time.

Microfossils are fossilized forms of microscopic life. Many microfossils are small (1 to 2 μm in diameter) and appear to be single-celled, lack external appendages, and have little evidence of internal structure. Thus, microfossils seem to resemble present-day prokaryotes.


Ever-Changing Life on Earth

Beginning with a single cell in the Archean eon, life has evolved into three monophyletic clades called domains: Eubacteria, Archaea, and Eukaryotes.


Compartmentalization of cells enabled the advent of eukaryotes

Members of the third great domain of life, the eukaryotes, appear in the fossil record much later, only about 1.5 bya. But despite the metabolic similarity of eukaryotic cells to prokaryotic cells, their structure and function enabled these cells to be larger, and eventually, allowed multicellular life to evolve.


Evolution of the endomembrane system

The hallmark of eukaryotes is complex cellular organization, highlighted by an extensive endomembrane system that subdivides the eukaryotic cell into functional compartments, including the nucleus (figure 26.9; see also chapter 4). The evolution of a nuclear membrane, not found in bacteria and archaea, accounts for increased complexity in eukaryotes. In eukaryotes, RNA transcripts from nuclear DNA are processed and transported across the nuclear membrane into the cytosol, where translation occurs. The physical separation of transcription and translation in eukaryotes adds additional levels of control to the process of gene expression.

The Golgi apparatus and endoplasmic reticulum are key innovations that facilitate intracellular transport and the localization of proteins in specific regions of the cell (see chapter 4). These membrane systems, as well as the nuclear membrane, arose through the infolding of the cellular membrane.


Endosymbiosis and the origin of eukaryotes

With few exceptions, modern eukaryotic cells possess the energy-producing organelles termed mitochondria, and photosynthetic eukaryotic cells possess chloroplasts, the energy-harvesting organelles. Mitochondria and chloroplasts are both believed to have entered early eukaryotic cells by a process called endosymbiosis, which is discussed in more detail in chapter 29.


Multicellularity leads to cell specialization

The unicellular body plan has been tremendously successful, with unicellular prokaryotes and eukaryotes constituting about half of the biomass on Earth. But a single cell has limits, even with the within-cell specialization provided by compartmentalization in eukaryotes. The evolution of multicellularity allowed organisms to deal with their environments in novel ways through differentiation of cell types into tissues and organs.

True multicellularity, in which the activities of individual cells are coordinated and the cells themselves are in contact, occurs only in eukaryotes and is one of their major characteristics.

Multicellularity has arisen independently in different eukaryotic supergroups. For example, multicellularity arose independently in the red, brown, and green algae. One lineage of multicellular green algae was the ancestor of the plants (see chapter 29). A different unicellular ancestor in the Opisthokonts gave rise to all multicellular animals.


Sexual reproduction increases genetic diversity

Sexual reproduction allows greater genetic diversity through the processes of meiosis and crossing over.


Major innovations allowed for the move onto land

The Cambrian radiation was confined to the ocean. Shortly after, plants and then animals colonized terrestrial environments. The evolution of photosynthesis, which resulted in an O2-rich atmosphere, also resulted in the ozone layer, which protects life on the surface from UV radiation.


The Miller–Urey experiment demonstrated that
life originated on Earth.
organic molecules could have originated in the early atmosphere.
the early genetic material on the planet was DNA.
the early atmosphere contained large amounts of oxygen.

organic molecules could have originated in the early atmosphere.


Plate tectonics can contribute to
volcanoes and earthquakes.
formation of supercontinents.
increased weathering and CO2 sequestration.
All of the choices are correct.

All of the choices are correct.


Identify which of the following statements is false and correct the statement.
Brown and red algae are not closely related phylogenetically.
Chloroplasts in brown and red algae are monophyletic.
Brown algae gained chloroplasts by engulfing green algae (endosymbiosis).
None of the statements are false.

Brown algae gained chloroplasts by engulfing green algae (endosymbiosis).

should be red algae.


Which of the following events occurred first in eukaryotic evolution?
Endosymbiosis and mitochondria evolution
Endosymbiosis and chloroplast evolution
Compartmentalization and formation of the nucleus
Formation of multicellular organisms

Compartmentalization and formation of the nucleus


Fossil data indicate that
life originated 4 bya.
life may have originated 3.5 bya, but definitely by 3.2 bya.
the Cambrian explosion led to the origin of life.
plants played an important role in causing glaciation.

life may have originated 3.5 bya, but definitely by 3.2 bya.


A global glaciation would be unlikely to occur if
a supercontinent formed near the equator and there was extensive rainfall.
millions of acres of forest were cleared.
vast amounts of phosphorous found its way into aquatic and oceanic environments.
there was a rapid expansion of algal populations in the ocean.

millions of acres of forest were cleared.


Although we do not know how early life arose, the following likely happened:
All organic molecules were transported to Earth by meteors.
High levels of oxygen were essential for glycolysis.
Lipids organized to form cell membranes.
Organic molecules formed once temperatures reached moderate levels, equivalent to today’s environment.

Lipids organized to form cell membranes.


Which bits of evidence would convince you that life originated as early as 3.2 bya?
You look at a microfossil under a scanning electron microscope and it is the same shape as a cell.
A high-quality transmission electron micrograph of a fossil cell reveals cellular compartments, including a possible nucleus.
Potassium dating of a fossil containing a possible cell indicates that the fossil is 3.2 billion years old.
Using transmission and scanning electron microscopy, you find evidence of a carbon-based material in what appears to be a cell wall of a fossil isotopically dated as 3.2 billion years old.

Using transmission and scanning electron microscopy, you find evidence of a carbon-based material in what appears to be a cell wall of a fossil isotopically dated as 3.2 billion years old.


During which times would you expect that geographic isolation would be particularly important in the evolution of life?
Cambrian period
End of the Paleozoic era
The beginning of the Cenozoic era
Both a and c are correct.

The beginning of the Cenozoic era.


The chloroplasts of brown algae
have a chromosome with a very different DNA sequence than the chromosome of a red alga.
are surrounded by four membranes.
are surrounded by two membranes.
have a chromosome with a DNA sequence that is similar to red algae, but very different from green algae.

are surrounded by two membranes.