Multicellularity Vs Unicellularity: Evolution From Unicellular to a Multicellular World.

Recently, I was rummaging through Quora, a question and answer social media site, and I came across a rather ostentatiously simple looking question.

A single cell performs many functions of life. Why do we need many cells?

You could find the answers by various Quorans over here.

At first, I thought, it’s such an easy-peasy question, why to hack my brains over it!

But as soon as I started gloating over it, enshrouded in a cloak of complacency, out of the blue! I had an epiphany.

Indeed, a unicellular organism (a single cell) can perform all the functions for a healthy existence then why did, during the course of evolution, more complex multicellular life forms arose and thrived. Not only that, in the billions of years of earth’s existence, even unicellular organisms evolved and thrived, so much so, that some of them even became a vehicle for mutations in multicellular organisms (certain viruses) inserting foreign genetic material in the host genome during an infection; thereby, advancing the cause of evolution.

Today, one cannot think of the existence of multicellularity without the support from the unicellular world.

Unicellular organisms are everywhere, with bacteria being the most abundant microorganisms in soil, with a population of 1010–1011 individuals and 6,000–50,000 species per gram of soil and a biomass of 40-500 grams per m2. [Soil Microorganisms: An Overview]

A_Novel_Representation_Of_The_Tree_Of_Life
A 2016 (metagenomic) representation of the tree of life. https://www.nature.com/articles/nmicrobiol201648

In 1859, Charles Darwin published On the Origin of Species in which he twice stated the hypothesis that there was only one progenitor for all life forms. In the summation, he states, “Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed. The very last sentence begins with a restatement of the hypothesis: There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one…Source

The last universal common ancestor (LUCA), also called the last universal ancestor (LUA), cenancestor, or (incorrectly) progenote, is the most recent population of organisms from which all organisms now living on Earth have a common descentLUCA is the most recent common ancestor of all current life on Earth. LUCA is not thought to be the first living organism on Earth, but only one of many early organisms, whereas the others became extinct.

In a gist, it was LUCA that gave rise to Bacteria, Archaea, and Eukaryotes.

And, if we happen to extrapolate some more information from the above findings, it could also mean that many unicellular organism species which ruled the earth during the beginning, too became extinct and gave rise to LUCA.

A simpler version of ‘tree of life’ is depicted below.

treeoflife
The Tree of Life http://epicofevolution.com/evolution.html

But a question again arises, what was the need to evolve and diversify?

To answer that we need to know some basics.

Definition of a Cell

(Science: Cell Biology) Cell – Biology-Online Dictionary

1. The structural, functional and biological unit of all organisms.

2. An autonomous self-replicating unit that may exist as the functional independent unit of life (as in the case of a unicellular organism), or as sub-unit in a multicellular organism (such as in plants and animals) that is specialized into carrying out particular functions towards the cause of the organism as a whole.

3. A membrane-bound structure containing biomolecules, such as nucleic acids, proteins, and polysaccharides.

There are two distinct types of cells: prokaryotic cells (e.g. bacterial cells) and eukaryotic cells (e.g. plant or animal cell). The main difference between the two is a well-defined nucleus surrounded by a membranous nuclear envelope present only in eukaryotic cells. Despite this difference they share a number of common features: the genetic information is stored in genes, proteins serve as their main structural material, ribosomes are used to synthesize proteins, adenosine triphosphate is the main source of metabolic energy to sustain various cellular processes, and a cell membrane that controls the flow of substances into and out of the cell.

Let us diverge a little and indulge ourselves in the realm of particle physics.

Particle physics (also high energy physics) is a branch of physics that studies the nature of the particles that constitute matter and radiation. Although the word particle can refer to various types of very small objects (e.g. protons, gas particles, or even household dust), particle physics usually investigates the irreducibly smallest detectable particles and the fundamental interactions necessary to explain their behaviour. By our current understanding, these elementary particles are excitations of the quantum fields that also govern their interactions. The currently dominant theory explaining these fundamental particles and fields, along with their dynamics, is called the Standard Model. Source

It is predicted, as per the Big-Bang nucleosynthesis (BBN), in the early universe, there was an abundance of the light elements, Deuterium (D), Tritium (3He), Helium (4He), and Lithium-7 (7Li), synthesized at the end of the ‘first three minutes’ of the creation of universe from subatomic particles. Later on, the heavy elements such as Carbon (C), Nitrogen (N), Oxygen (O), and Iron (Fe) (“metals”) were forged in the process of stellar nucleosynthesis i.e. under extremes of pressure and temperatures inside stars. Source

So, even in the field of physics, for complexity to arise, a balance of certain parameters is an essential prerequisite, as well as, the presence of simple elements to be forged into the newer complex elements.

Similarly, in the early stages of the development of planet Earth, billions of years ago, under the extremes of conditions, simple life forms, capable of confronting such unrelenting conditions came into being.

As per our current knowledge, none of the planets in our solar system are capable of supporting life, except out Earth. But the conditions on Earth were not always so suitable for life.

The video can be viewed here.

Earth’s long history tells a story of constant environmental change and of close connections between physical and biological environments. It also demonstrates the robustness of life. Simple organisms first appeared on Earth some 3.8 billion years ago, and complex life forms emerged approximately 2 billion years ago. Life on Earth has endured through many intense stresses, including ice ages, warm episodes, high and low oxygen levels, mass extinctions, huge volcanic eruptions, and meteorite impacts. Untold numbers of species have come and gone, but life has survived even the most extreme fluxes. Source

How unique are the conditions that allowed life to develop and diversify on Earth? Some scientists contend that circumstances on Earth were extremely unusual and that complex life is very unlikely to find such favourable conditions elsewhere in our universe, although simple life forms like microbes may be very common [2]. Other scientists believe that Earth’s history may not be the only environment in which life could develop and that other planets with very different sets of conditions could foster complex life. What is generally agreed, however, is that no other planet in our solar system has developed along the same geologic and biologic path as Earth. Life as we know it is a direct result of specific conditions that appear thus far to be unique to our planet.

geologic_time
© United States Geological Survey. The geologic time scale divides Earth’s history into blocks of time that are ordered sequentially and read from bottom to top, like the rock records on which it is based. https://www.learner.org/courses/envsci/visual/visual.php?shortname=geologic_time

Until the early twentieth century, researchers could only assign relative ages to geologic records. More recently, the expanding field of nuclear physics has enabled scientists to calculate the absolute age of rocks and fossils using radiometric dating, which measures the decay of radioactive isotopes in rock samples. This approach has been used to determine the ages of rocks more than 3.5 billion years old [3]. Once they establish the age of multiple formations in a region, researchers can correlate strata among those formations to develop a fuller record of the entire area’s geologic history.

layers_from_parks
The geologic history of southern California. © United States Geological Survey, Western Earth Surface Processes Team. Scientists study characteristics of a geologic stratum such as colour and mineral type so that they can match up portions of that stratum that are exposed in separate regions. Strata can extend for many miles, although they may only be exposed in a few places. https://www.learner.org/courses/envsci/visual/visual.php?shortname=layers_from_parks

The young Earth was anything but habitable. Radioactive elements decaying within its mass and impacts from debris raining down from space generated intense heat—so strong that the first eon of Earth’s history, from about 4.5 to 3.8 billion years ago, is named the Hadean after Hades, the Greek word for hell. Most original rock from this period was melted and recycled into Earth’s crust, so very few samples remain from our planet’s formative phase. But by studying meteorites—stony or metallic fragments up to 4.5 billion years old that fall to Earth from space—scientists can see what materials were present when the solar system was formed and how similar materials may have been melted, crystallized and transformed as Earth took shape. Source

Our understanding of Earth’s history and the emergence of life draws on other scientific fields along with geology and palaeontology. Biologists trace genealogical relationships among organisms and the expansion of biological diversity. And climate scientists analyze changes in Earth’s atmosphere, temperature patterns, and geochemical cycles to determine why events such as ice ages and rapid warming events occurred. All of these perspectives are relevant because organisms and the physical environment on Earth have developed together and influenced each other’s evolution in many ways. Source

How did early Earth transition from a hell-like environment to temperatures more hospitable to life? Early in the Archean (ancient) eon, about 3.8 billion years ago, the rain of meteors and rock bodies from space ended, allowing our planet’s surface to cool and solidify. Water vapour in the atmosphere condensed and fell as rain, creating oceans. These changes created the conditions for geochemical cycling—flows of chemical substances between reservoirs in Earth’s atmosphere, hydrosphere (water bodies), and lithosphere (the solid part of Earth’s crust).

At this time the sun was about 30 per cent dimmer than it is today, so our planet received less solar radiation. Earth’s surface should have been well below the freezing point of water, too cold for life to exist, but evidence shows that liquid water was present and that simple life forms appeared as far back as 3.5 billion years ago. This contradiction is known as the “faint young sun” paradox. The unexpected warmth came from greenhouse gases in Earth’s atmosphere, which retained enough heat to keep the planet from freezing over. Source

young_sun
Based on the strength of the young sun, the early Earth should have been frozen over. However, greenhouse gases in Earth’s atmosphere retained enough heat to provide liquid water on Earth’s surface. https://www.learner.org/courses/envsci/visual/visual.php?shortname=young_sun

The Archean atmosphere was a mix of gases including nitrogen, water vapor, methane (CH4), and CO2. (As discussed in section 6, “Atmospheric Oxygen,” free oxygen did not accumulate in the atmosphere until more than two billion years after Earth was formed.) Volcanoes emitted CO2 as a byproduct of heating within the Earth’s crust. But instead of developing a runaway greenhouse effect like that on Venus, Earth’s temperatures remained within a moderate range because the carbon cycle includes a natural sink—a process that removes excess carbon from the atmosphere. This sink involves the weathering of silicate rocks, such as granites and basalts, that make up much of Earth’s crust.

geochemical_carbon (1)
The geochemical carbon cycle © Snowball Earth.org. A silicate weathering feedback adjusts atmospheric CO2 sources and sinks. Plate tectonics recycle sedimentary carbon (carbonate and organic matter) as volcanic and metamorphic CO2. https://www.learner.org/courses/envsci/visual/visual.php?shortname=geochemical_carbon

We can see how durable Earth’s silicate weathering “thermostat” is by looking at some of the most extreme climate episodes on our planet’s history: severe glaciations that occurred during the Proterozoic era. The first “Snowball Earth” phase is estimated to have occurred about 2.3 billion years ago, followed by several more between about 750 and 580 million years ago. Proponents of the Snowball Earth theory believe that Earth became so cold during several glacial cycles in this period that it essentially froze over from the equator to the poles for spans of ten million years or more. But ultimately, they contend, the carbon-silicate cycle freed Earth from this deep-freeze state.

If Earth has a natural thermostat, how could it become cold enough for the entire planet to freeze over? One possible cause is continental drift. Researchers believe that around 750 million years ago, most of the continents may have been clustered in the tropics following the breakup of the supercontinent Rodinia, and that such a configuration would have had pronounced effects on Earth’s climate [4].

How could life survive a snowball episode?

Paradoxically, scientists theorize that these deep freezes may have indirectly spurred the development of complex life forms. The most complex life forms on Earth at the time of the Neoproterozoic glaciations were primitive algae and protozoa. Most of these existing organisms were undoubtedly wiped out by glacial episodes. But recent findings have shown that some microscopic organisms can flourish in extremely challenging conditions—for example, within the channels inside floating sea ice and around vents on the ocean floor where superheated water fountains up from Earth’s mantle. These environments may have been the last reservoirs of life during Snowball Earth phases. Even a small amount of geothermal heat near any of the tens of thousands of natural hot springs that exist on Earth would have been sufficient to create small holes in the ice. And those holes would have been wonderful refuges where life could persist.

Organisms adaptable enough to survive in isolated environments would have been capable of rapid genetic evolution in a short time. The last hypothesized Snowball Earth episode ended just a few million years before the Cambrian explosion, an extraordinary diversification of life that took place from 575 to 525 million years ago. It is possible, although not proven, that the intense selective pressures of snowball glaciations may have fostered life forms that were highly adaptable and ready to expand quickly once conditions on Earth’s surface moderated.

A stable climate is only one key requirement for the complex life forms that populate Earth today. Multi-cellular organisms also need a ready supply of oxygen for respiration.

Today oxygen makes up about 20 per cent of Earth’s atmosphere, but for the first two-billion-years after Earth formed, its atmosphere was anoxic (oxygen-free). About 2.3 billion years ago, oxygen increased from a trace gas to perhaps one per cent of Earth’s atmosphere. Another jump took place about 600 million years ago, paving the way for multi-cellular life forms to expand during the Cambrian Explosion.

Oxygen is a highly reactive gas that combines readily with other elements like hydrogen, carbon, and iron. Many metals react directly with oxygen in the air to form metal oxides. For example, rust is an oxide that forms when iron reacts with oxygen in the presence of water. This process is called oxidation, a term for reactions in which a substance loses electrons and become more positively charged. In this case, iron loses electrons to oxygen (Fig. 12). What little free oxygen was produced in Earth’s atmosphere during the Archean eon would have quickly reacted with other gases or with minerals in surface rock formations, leaving none available for respiration.

oxidation
Oxidation of iron to form rust. In oxidation reactions such as rust formation, a molecule or atom loses electrons and becomes more positively charged. Although it can also take place with other chemicals, oxidation is so named because the first such reactions to be thoroughly studied were reactions with oxygen. https://www.learner.org/courses/envsci/visual/visual.php?shortname=oxidation

Why did oxygen levels rise? 

Cyanobacteria, the first organisms capable of producing oxygen through photosynthesis, emerged well before the first step up in atmospheric oxygen concentrations, perhaps as early as 2.7 billion years ago. Their oxygen output helped to fill up the chemical sinks, such as iron in soils, that removed oxygen from the air. But plant photosynthesis alone would not have provided enough oxygen to account for this increase, because heterotrophs (organisms that are not able to make their own food) respire oxygen and use it to metabolize organic material. If all new plant growth is consumed by animals that feed on living plants and decomposers that break down dead plant material, carbon and oxygen cycle in what is essentially a closed loop and net atmospheric oxygen levels remain unchanged.

cycling
Cycling of carbon and oxygen. Photosynthesis removes CO2 from the air and adds oxygen, while cellular respiration removes oxygen from the air and adds CO2. The processes generally balance each other out. https://www.learner.org/courses/envsci/visual/visual.php?shortname=cycling

However, the material can leak out of this loop and alter carbon-oxygen balances. If organic matter produced by photosynthesis is buried in sediments before it decomposes (for example, dead trees may fall into a lake and sink into the lake bottom), it is no longer available for respiration. The oxygen that decomposers would have consumed as they broke it down goes unused, increasing atmospheric oxygen concentrations. Many researchers theorize that this process caused the initial rise in atmospheric oxygen.

oxygen_levels
Atmospheric oxygen levels over geological time. © Snowball Earth.org Increases in atmospheric oxygen levels are closely correlated with hypothesized Snowball Earth episodes and with the appearance of increasingly complex life forms. https://www.learner.org/courses/envsci/visual/visual.php?shortname=oxygen_levels

Why have atmospheric oxygen levels stayed relatively stable since this second jump?

As discussed above, the carbon-oxygen cycle is a closed system that keeps levels of both elements fairly constant. The system contains a powerful negative feedback mechanism, based on the fact that most animals need oxygen for respiration. If atmospheric oxygen levels rose substantially today, marine zooplankton would eat and respire organic matter produced by algae in the ocean at an increased rate, so a lower fraction of organic matter would be buried, cancelling the effect. Falling oxygen levels would reduce feeding and respiration by zooplankton, so more of the organic matter produced by algae would end up in sediments and oxygen would rise again. Fluctuations in either direction thus generate changes that push oxygen levels back toward a steady state.

Forest fires also help to keep oxygen levels steady through a negative feedback. Combustion is a rapid oxidation reaction, so increasing the amount of available oxygen will promote a bigger reaction. Rising atmospheric oxygen levels would make forest fires more common, but these fires would consume large amounts of oxygen, driving concentrations back downward.

We do not know exactly when life first appeared on Earth. There is clear evidence that life existed at least 3 billion years ago, and the oldest sediments that have been discovered—rocks formed up to 3.8 billion years ago—bear marks that some scientists believe could have been left by primitive microorganisms. If this is true it tells us that life originated soon after the early period of bombardment by meteorites, on an Earth that was probably much warmer than today and had only traces of oxygen in its atmosphere.

life_tree
The universal Tree of Life © Source: Jack D. Farmer, 2000. Hydrothermal Systems: Doorways to Early Biosphere Evolution, GSA Today 10(7), 1-9. As the Tree of Life shows, microbes were the dominant early life forms on Earth. https://www.learner.org/courses/envsci/visual/visual.php?shortname=life_tree

For the first billion years or so, life on Earth consisted of bacteria and archaea, microscopic organisms that represent two of the three genealogical branches on the Tree of Life. Both groups are prokaryotes (single-celled organisms without nuclei). Archaea were recognized as a unique domain of life in the 1970s, based on some distinctive chemical and genetic features. The order in which branches radiate from the Tree of Life shows the sequence in which organisms evolved. Reading from the bottom up in the same way in which a tree grows and branches outward, we can see that eucarya (multi-celled animals) were the last major group to diverge and that animals are among the newest subgroups within this domain.

Life on Earth existed for many millions of years without atmospheric oxygen. The lowest groups on the Tree of Life, including thermatogales and nearly all of the archaea, are anaerobic organisms that cannot tolerate oxygen. Instead they use hydrogen, sulfur, or other chemicals to harvest energy through chemical reactions. These reactions are key elements of many chemical cycles on Earth, including the carbon, sulfur, and nitrogen cycles.

“Prokaryotic metabolisms form the fundamental ecological circuitry of life,”

writes paleontologist Andrew Knoll. [5]

“Bacteria, not mammals, underpin the efficient and long-term functioning of the biosphere” [5]

Some bacteria and archaea are extremophiles—organisms that thrive in highly saline, acidic, or alkaline conditions or other extreme environments, such as the hot water around hydrothermal vents in the ocean floor. Early life forms’ tolerances and anaerobic metabolisms indicate that they evolved in very different conditions from today’s environment.

Microorganisms are still part of Earth’s chemical cycles, but most of the energy that flows through our biosphere today comes from photosynthetic plants that use light to produce organic material. When did photosynthesis begin? Archaean rocks from western Australia that have been dated at 3.5 billion years old contain organic material and fossils of early cyanobacteria, the first photosynthetic bacteria [6]. These simple organisms jump-started the oxygen revolution by producing the first traces of free oxygen through photosynthesis: Knoll calls them “the working-class heroes of the Precambrian Earth.” [7]

Cyanobacteria are widely found in tidal flats, where the organic carbon that they produced was buried, increasing atmospheric oxygen concentrations. Mats of cyanobacteria and other microbes trapped and bound sediments, forming wavy structures called stromatolites (layered rocks) that mark the presence of microbial colonies.

The third domain of life, eukaryotes, are organisms with one or more complex cells. A eukaryotic cell contains a nucleus surrounded by a membrane that holds the cell’s genetic material. Eukaryotic cells also contain organelles—sub-components that carry out specialized functions such as assembling proteins or digesting food. In plant and eukaryotic algae cells, chloroplasts carry out photosynthesis. These organelles developed through a process called endosymbiosis in which cyanobacteria took up residence inside host cells and carried out photosynthesis there. Mitochondria, the organelles that conduct cellular respiration (converting energy into usable forms) in eukaryotic cells, are also descended from cyanobacteria.

The first eukaryotic cells evolved sometime between 1.7 and 2.5 billion years ago, perhaps coincident with the rise in atmospheric oxygen around 2.3 billion years ago. As the atmosphere and the oceans became increasingly oxygenated, organisms that used oxygen spread and eventually came to dominate Earth’s biosphere. Chemosynthetic organisms remained common but retreated into sediments, swamps, and other anaerobic environments.

Throughout the Proterozoic era, from about 2.3 billion years ago until around 575 million years ago, life on Earth was mostly single-celled and small. Earth’s biota consisted of bacteria, archaea, and eukaryotic algae. Food webs began to develop, with amoebas feeding on bacteria and algae. Earth’s land surfaces remained harsh and largely barren because the planet had not yet developed a protective ozone layer (this screen formed later as free oxygen increased in the atmosphere), so it was bombarded by intense ultraviolet radiation. However, even shallow ocean waters shielded microorganisms from damaging solar rays, so most life at this time was aquatic.

Global glaciations occurred around 2.3 billion years ago and again around 600 million years ago. Many scientists have sought to determine whether there is a connection between these episodes and the emergence of new life forms around the same times. For example, one Snowball Earth episode about 635 million years ago is closely associated with the emergence of multicellularity in microscopic animals [8]. However, no causal relationship has been proved.

The first evidence of multicellular animals appears in fossils from the late Proterozoic era, about 575 million years ago, after the last snowball glaciation. These impressions were made by soft-bodied organisms such as worms, jellyfish, sea pens, and polyps similar to modern sea anemones. In contrast to the microorganisms that dominated the Proterozoic era, many of these fossils are at least several centimetres long, and some measure up to a meter across.

fossils
Fossils of Kimberella (thought to be a jellyfish). Courtesy Wikimedia Commons. GNU Free Documentation License. Kimberella was originally thought to be a type of jellyfish, but scientists now believe that it had rigid parts (probably a hard, shell-like covering) which formed the deep depressions in these fossils. https://www.learner.org/courses/envsci/visual/visual.php?shortname=fossils

Shortly after this time, starting about 540 million years ago, something extraordinary happened: the incredible diversification of complex life known as the Cambrian Explosion. Within 50 million years every major animal phylum known in fossil records quickly appeared. The Cambrian Explosion can be thought of as multicellular animals’ “big bang”—an incredible radiation of complexity.

What triggered the Cambrian Explosion?

Scientists have pointed to many factors. For example, the development of predation probably spurred the evolution of shells and armour, while the growing complexity of ecological relationships created distinct roles for many sizes and types of organisms. Rising atmospheric and oceanic oxygen levels promoted the development of larger animals, which need more oxygen than small ones in order to move blood throughout their bodies. And some scientists believe that a mass extinction at the end of the Proterozoic era created a favourable environment for new life forms to evolve and spread.

Following the Cambrian Explosion, life diversified in several large jumps that took place over three eras: Paleozoic, Mesozoic, and Cenozoic (referring back to Fig. 4, the Cambrian period was the first slice of the Paleozoic era). Together these eras make up the Phanerozoic eon, a name derived from the Greek for “visible life.” The Phanerozoic, which runs from 540 million years ago to the present, has also been a tumultuous phase in the evolution of life on Earth, with mass extinctions at the boundaries between each of its three geologic eras. Figure 18 shows the scale of historic mass extinctions as reflected in marine fossil records.

Early in the Paleozoic most of Earth’s fauna lived in the sea. Many Cambrian organisms developed hard body parts like shells and bones, so fossil records became much more abundant and diverse. The Burgess Shale, rock beds in British Columbia made famous in palaeontologist Stephen Jay Gould’s book Wonderful Life, are ancient reef beds in the Canadian Rockies of British Columbia that are filled with fossil deposits from the mid-Cambrian period [9].

Land plants emerged between about 500 and 400 million years ago. Once established, they stabilized soil against erosion and accelerated the weathering of rock by releasing chemicals from their roots. Since faster weathering pulls increased amounts of carbon out of the atmosphere, plants reduced the greenhouse effect and cooled Earth’s surface so dramatically that they are thought to have helped cause several ice ages and mass extinctions during the late Devonian period, about 375 million years ago. By creating shade, they also provided habitat for the first amphibians to move from water to land.

The most severe of all mass extinctions took place at the end of the Paleozoic era at the Permian/Triassic boundary, wiping out an estimated 80 to 85 per cent of all living species. Scientists still do not understand what caused this crisis. Geologic records indicate that deep seas became anoxic, which suggest that something interfered with normal ocean mixing and that Earth’s climate suddenly became much warmer and drier. Possible causes for these developments include massive volcanic eruptions or a melting of methane hydrate deposits (huge reservoirs of solidified methane), both of which could have sharply increased the greenhouse effect.

The Mesozoic era, spanning the Triassic, Jurassic, and Cretaceous periods, was the era of reptiles, which colonized land and air more thoroughly than the amphibians that preceded them out of the water. Dinosaurs evolved in the Triassic, about 215 million years ago, and became the largest and most dominant animals on Earth for the next 150 million years. This period also saw the emergence of modern land plants, including the first angiosperms (flowering plants); small mammals; and the first birds, which evolved from dinosaurs. Figure 19 shows a model of a fossilized Archaeopteryx, a transitional species from the Jurassic period with both avian and dinosaur features.

archaeopteryx
Model of Archaeopteryx fossil. © Wikimedia Commons. CeCILL license. Archaeopteryx had some features not seen in today’s birds, such as a long bony tail and a full set of teeth. However, it also had feathers, wings, and reduced fingers, which are characteristic of modern birds. https://www.learner.org/courses/envsci/visual/visual.php?shortname=archaeopteryx

Another mass extinction at the end of the Mesozoic, 65 million years ago, killed all of the dinosaurs except for birds, along with many other animals. For many years scientists thought that climate change caused this extinction, but in 1980 physicist Louis Alvarez, his son, Walter, a geologist, and other colleagues published a theory that a huge meteorite had hit Earth, causing impacts like shock waves, severe atmospheric disturbances, and a global cloud of dust that would have drastically cooled the planet. Their most important evidence was widespread deposits of iridium—a metal that is extremely rare in Earth’s crust but that falls to Earth in meteorites—in sediments from the so-called K-T (Cretaceous-Tertiary) boundary layer.

Further evidence discovered since 1980 supports the meteor theory, which is now widely accepted. A crater has been identified at Chicxulub, in Mexico’s Yucatan peninsula, that could have been caused by a meteorite big enough to supply the excess iridium, and grains of shocked quartz from the Chicxulub region have been found in sediments thousands of kilometres from the site that date to the K-T boundary era.

The first mammals on Earth were rodent-sized animals that evolved in the shadow of dinosaurs during the Jurassic and Triassic periods. After the K-T boundary extinction eliminated dinosaurs as predators and competitors, mammals radiated widely. Most of the modern mammal orders, from bats to large types like primates and whales, appeared within about 10 million years after dinosaurs died out. Because mammals could maintain a relatively constant internal temperature in hot or cold environments, they were able to adapt to temperature changes more readily than cold-blooded animals like reptiles, amphibians, and fish. This characteristic helped them to populate a wide range of environments.

Another important ecological shift was the spread of angiosperms (flowering plants), which diversified and became the dominant form of land plants. Unlike earlier plants like ferns and conifers, angiosperms’ seeds were enclosed within a structure (the flower) that protected developing embryos. Their flower petals and fruits, which grew from plants’ fertilized ovaries, attracted animals, birds, and insects that helped plants spread by redistributing pollen and seeds. These advantages enabled angiosperms to spread into more diverse habitats than earlier types of plants.

Earth’s climate continued to fluctuate during the Cenozoic, posing challenges for these new life forms. After an abrupt warming about 55 million years ago, the planet entered a pronounced cooling phase that continued up to the modern era. One major cause was the ongoing breakup of Gondwanaland, a supercontinent that contained most of the landmasses in today’s southern hemisphere, including Africa, South America, Australia, and India.

Once these fragments started to separate about 160 million years ago, ocean currents formed around Antarctica. Water trapped in these currents circulated around the pole and became colder and colder. As a result, Antarctica cooled and developed a permanent ice cover, which in turn cooled global atmospheric and ocean temperatures. Climates became dryer, with grasslands and arid habitat spreading into many regions that previously had been forested.

Continued cooling through the Oligocene and Miocene eras, from about 35 million to 5 million years ago, culminated in our planet’s most recent ice age: a series of glacial advances and retreats during the Pleistocene era, starting about 3.2 million years ago. During the last glacial maximum, about 20,000 years ago, ice sheets covered most of Canada and extended into what is now New England and the upper Midwestern states.

Humans are latecomers in geologic time: when Earth’s history is mapped onto a 24-hour timescale, we appear less than half a minute before the clock strikes midnight. [1]

Human evolution occurred roughly in parallel with the modern ice age and was markedly influenced by geologic and climate factors. Early hominids (members of the biological family of the great apes) radiated from earlier apes in Africa between 5 and 8 million years ago. Humans’ closest ancestor, Australopithecus, was shorter than modern man and is thought to have spent much of its time living in trees. The human genus, Homo, which evolved about 2.5 million years ago, had a larger brain, used hand tools, and ate a diet heavier in meat than Australopithecus. In sum, Homo was better adapted for life on the ground in a cooler, drier climate where forests were contracting and grasslands were expanding.

By 1.9 million years ago, Homo erectus had migrated from Africa to China and Eurasia, perhaps driven partly by climate shifts and resulting changes to local environments.Homo sapiens, the modern human species, is believed to have evolved in Africa about 200,000 years ago. Homo sapiens gradually migrated outward from Africa, following dryland migration routes that were exposed as sea levels fell during glacial expansions. By about 40,000 years ago Homo sapiens had settled Europe, and around 10,000 years ago man reached North America. Today, archaeologists, anthropologists, and geneticists are working to develop more precise maps and histories of the human migration out of Africa, using mitochondrial DNA (maternally inherited genetic material) to assess when various areas were settled.

Early in their history, humans found ways to manipulate and affect their environment. Mass extinctions of large mammals, such as mammoths and saber-toothed cats, occurred in North and South America, Europe, and Australia roughly when humans arrived in these areas. Some researchers believe that over-hunting, alone or in combination with climate change, may have been the cause. After humans depleted wildlife, they went on to domesticate animals, clear forests, and develop agriculture, with steadily expanding impacts on their surroundings.

All the information in the above blog has been sourced from http://www.learner.org: Many Planets, One Earth

So, from the above discussion, it can easily be surmised that the world that exists today has not appeared by some magic in a matter of few seconds but is a result of billions of years of natural selection and climate change.

The life on Earth started with unicellular organisms. These organisms evolved with changing environment which was brought about by geologic factors as well as the organisms themselves. Evolution is a continuous process, even if the environment remains static the process of evolutions strives to create species better suited to that particular niche.

So, unicellular organisms not only formed the basis for further development but also modulated evolution of multicellular organisms. Therefore, comparing one with the other would not be wise, as both have a niche of their own.

Reference:

      1. American Museum of Natural History, “Our Dynamic Planet: Rock Around the Clock,” http://www.amnh.org/education/resources/rfl/web/earthmag/peek/pages/clock.htm.
      1. Peter D. Ward and Donald Brownlee, Rare Earth: Why Complex Life Is Uncommon in the Universe (New York: Springer-Verlag, 2000)
      1. U.S. Geological Survey, “Radiometric Time Scale,” http://pubs.usgs.gov/gip/geotime/radiometric.html, and “The Age of the Earth,” http://geology.wr.usgs.gov/parks/gtime/ageofearth.html.
      1. Paul F. Hoffman and Daniel P. Schrag, “Snowball Earth,” Scientific American, January 2000, pp. 68–75.
      1. Andrew Knoll, Life on a Young Planet: The First Three Billion Years of Evolution on Earth (Princeton University Press, 2003), p. 23.
      1. University of California Museum of Paleontology, “Cyanobacteria: Fossil Record,” http://www.ucmp.berkeley.edu/bacteria/cyanofr.html.
      1. Knoll, Life on a Young Planet, p. 42.
      1. “Did the Snowball Earth Kick-Start Complex Life?”, http://www.snowballearth.org/kick-start.html.
    1. Stephen Jay Gould, Wonderful Life: The Burgess Shale and the Nature of History (New York: Norton, 1990).

Further Reading 

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