BSC 1011C
General Biology II
Dr.
Graeme Lindbeck
glindbeck@valenciacollege.edu
BSC 1011C
General Biology II
Dr.
Graeme Lindbeck
glindbeck@valenciacollege.edu
Life is a continuum extending from the earliest organisms through various phylogenetic branches to the great variety of forms alive today.
The diversification of life on Earth began over 3.8 billion ago.
Geologic events that alter environments have changed the course of biological evolution.
Conversely, life has changed the planet it inhabits.
Historical study of any sort is an inexact discipline that depends on the preservation, reliability, and interpretation of past records.
One can view the chronology of the major episodes that shaped life as a phylogenetic tree.
Alternatively, we can view these episodes with a clock analogy.
For the first three-quarters of evolutionary history, Earth's only organisms were microscopic and mostly unicellular.
The oldest fossils that have been uncovered were embedded in rocks from western Australia that are 3.5 billion years ago.
Prokaryotes dominated evolutionary history from about 3.5 to 2.0 billion years ago.
Relatively early, prokaryotes diverged into two main evolutionary branches, the bacteria and the archaea.
Two rich sources for early prokaryote fossils are stromatolites (fossilized layered microbial mats) and sediments from ancient hydrothermal vent habitats.
Photosynthesis probably evolved very early in prokaryotic history.
Cyanobacteria, photosynthetic organisms that split water and produce O2 as a byproduct, evolved over 2.7 billion years ago.
While oxygen accumulation was gradual between 2.7 and 2.2 billion years ago, it shot up to 10% of current values shortly afterward.
This "corrosive" O2 had an enormous impact on life, dooming many prokaryote groups.
Other species evolved mechanisms to use O2 in cellular respiration, which uses oxygen to help harvest the energy stored in organic molecules.
Eukaryotic cells are generally larger and more complex than prokaryotic cells.
In part, this is due to the apparent presence of the descendents of "endosymbiotic prokaryotes" that evolved into mitochondria and chloroplasts.
While there is some evidence of earlier eukaryotic fossils, the first clear eukaryote appeared about 2.1 billion years ago.
This places the earliest eukaryotes at the same time as the oxygen revolution that changed the Earth's environment so dramatically.
A great range of eukaryotic unicellular forms evolved into the diversity of present-day "protists."
Multicellular organisms, differentiating from a single-celled precursor, appear 1.2 billion years ago as fossils, or perhaps as early as 1.5 billion years ago from molecular clock estimates.
Recent fossils finds from China have produced a diversity of algae and animals from 570 million years ago, including beautifully preserved embryos.
Geologic evidence for a severe ice age ("snowball Earth" hypothesis) from 750 to 570 million years ago may be responsible for the limited diversity and distribution of multicellular eukaryotes until the very late Precambrian.
A second radiation of eukaryotic forms produced most of the major groups of animals during the early Cambrian period.
Cnidarians (the plylum that includes jellies) and poriferans (sponges) were already present in the late Precambrian.
However, most of the major groups (phyla) of animals make their first fossil appearances during the relatively short span of the Cambrian period's first 20 million years.
The colonization of land was one of the pivotal milestones in the history of life.
Sometime between about 4.0 billion years ago, when the Earth's crust began to solidify, and 3.5 billion years ago when stromatolites appear, the first organisms came into being.
But science seeks natural causes for natural phenomena.
Most scientists favor the hypothesis that life on Earth developed from nonliving materials that became ordered into aggregates that were capable of self-replication and metabolism.
From the time of the Greeks until the 19th century, it was common "knowledge" that life could arise from nonliving matter, an idea called spontaneous generation.
In 1862, Louis Pasteur conducted broth experiments that rejected the idea of spontaneous generation even for microbes.
A sterile broth would "spoil" only if microorganisms could invade from the environment.
All life today arises only by the reproduction of preexisting life, the principle of biogenesis.
Although there is no evidence that spontaneous generation occurs today, conditions on the early Earth were very different.
One credible hypothesis is that chemical and physical processes in Earth's primordial environment eventually produced simple cells.
Under one hypothetical scenario this occurred in four stages:
This hypothesis leads to predictions that can be tested in the laboratory.
In the 1920's, A.I. Oparin and J.B.S. Haldane independently postulated that conditions on the early Earth favored the synthesis of organic compounds from inorganic precursors.
The reducing environment in the early atmosphere would have promoted the joining of simple molecules to form more complex ones.
The considerable energy required to make organic molecules could be provided by lightning and the intense UV radiation that penetrated the primitive atmosphere.
In 1953, Stanley Miller and Harold Urey tested the Oparin-Haldane hypothesis by creating, in the laboratory, the conditions that had been postulated for early Earth.
They discharged sparks in an "atmosphere" of gases and water vapor.
The Miller-Urey experiments produced a variety of amino acids and other organic molecules.
Other attempts to reproduce the Miller-Urey experiment with other gas mixtures also produced organic molecules, although in smaller quantities.
The Miller-Urey experiments still stimulate debate on the origin of Earth's early stockpile of organic ingredients.
The abiotic origin hypothesis predicts that monomers should link to form polymers without enzymes and other cellular equipment.
Researchers have produced polymers, including polypeptides, after dripping solutions of monomers onto hot sand, clay, or rock.
Life is defined partly by inheritance.
Today, cells store their genetic information as DNA, transcribe select sections into RNA, and translate the RNA messages into enzymes and other proteins.
Many researchers have proposed that the first hereditary material was RNA, not DNA.
Short polymers of ribonucleotides can be synthesized abiotically in the laboratory.
In the 1980's Thomas Cech discovered that RNA molecules are important catalysts in modern cells.
RNA catalysts, called ribozymes, remove introns from RNA.
Ribozymes also help catalyze the synthesis of new RNA polymers.
In the pre-biotic world, RNA molecules may have been fully capable of ribozyme-catalyzed replication.
Laboratory experiments have demonstrated that RNA sequences can evolve in abiotic conditions.
RNA molecules have both a genotype (nucleotide sequence) and a phenotype (three dimensional shape) that interacts with surrounding molecules.
Under particular conditions, some RNA sequences are more stable and replicate faster and with fewer errors than other sequences.
RNA-directed protein synthesis may have begun as weak binding of specific amino acids to bases along RNA molecules, which functioned as simple templates holding a few amino acids together long enough for them to be linked.
If RNA synthesized a short polypeptide that behaved as an enzyme helping RNA replication, then early chemical dynamics would include molecular cooperation as well as competition.
Living cells may have been preceded by protobionts, aggregates of abiotically produced molecules.
Protobionts do not reproduce precisely, but they do maintain an internal chemical environment from their surroundings and may show some properties associated with life, metabolism, and excitability.
In the laboratory, droplets of abiotically produced organic compounds, called liposomes, form when lipids are included in the mix.
The lipids form a molecular bilayer at the droplet surface, much like the lipid bilayer of a membrane.
Liposomes behave dynamically, growing by engulfing smaller liposomes or "giving birth" to smaller liposomes.
If enzymes are included among the ingredients, they are incorporated into the droplets.
The protobionts are then able to absorb substrates from their surroundings and release the products of the reactions catalyzed by the enzymes.
Unlike some laboratory models, protobionts that formed in the ancient seas would not have possessed refined enzymes, the products of inherited instructions
Once primitive RNA genes and their polypeptide products were packaged within a membrane, the protobionts could have evolved as units.
Molecular cooperation could be refined because favorable components were concentrated together, rather than spread throughout the surroundings.
As an example: suppose that an RNA molecule ordered amino acids into a primitive enzyme that extracted energy from inorganic sulfur compounds taken up from the surroundings
The most successful protobionts would grow and split, distributing copies of their genes to offspring.
Even if only one such protobiont arose initially by the abiotic processes that have been described, its descendents would vary because of mutation, errors in copying RNA.
Evolution via differential reproductive success of varied individuals presumably refined primitive metabolism and inheritance.
Laboratory simulations cannot prove that these kinds of chemical processes actually created life on the primitive Earth.
They describe steps that could have happened.
The origin of life is still subject to much speculation and alternative views.
Major debates also concern where life evolved.
The prevailing site until recently was in shallow water or moist sediments.
Modern phylogenetic analyses indicate that the ancestors of modern prokaryotes thrived in very hot conditions and may have lived on inorganic sulfur compounds that are common in deep-sea vent environments.
As understanding of our solar system has improved, the hypothesis that life is not restricted to Earth has received more attention.
Debate about the origin of terrestrial and extraterrestrial life abounds.
Traditionally, systematists have considered kingdom as the highest taxonomic category.
As a product of a long tradition, beginning with Linnaeus organisms were divided into only two kingdoms of life - animal or plant.
In 1969, R.H Whittaker argued for a five-kingdom system: Monera, Protista, Plantae, Fungi, and Animalia.
The five-kingdom system recognizes that there are two fundamentally different types of cells: prokaryotic (the kingdom Monera) and eukaryotic (the other four kingdoms).
Three kingdoms of multicellular eukaryotes were distinguished by nutrition, in part.
In Whittaker's system, the Protista consisted of all eukaryotes that did not fit the definition of plants, fungi, or animals.
During the last three decades, systematists applying cladistic analysis, including the construction of cladograms based on molecular data, have been identifying problems with the five-kingdom system.
Many microbiologists have divided the two prokaryotic domains into multiple kingdoms based on cladistic analysis of molecular data.
A second challenge to the five kingdom system comes from systematists who are sorting out the phylogeny of the former members of the kingdom Protista.
Clearly, taxonomy at the highest level is a work in progress.
There will be much more research before there is anything close to a new consensus for how the three domains of life are related and how many kingdoms there are.
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