Post n. 41 English
The history of the search for ancient
microfossils originated at the beginning of the last century when the famous
paleontologist C. D Walcott, of whom we will have occasion to speak later,
hypothesized that very ancient rocks called stromatolites were sedimentary
structures of biological origin.
The discovery by Elso S. Barghoorn, "I
fossili più antichi" Le Scienze 1971, of ancient microorganisms fossilized
in stromatolites of continental shields,
initiated a large number of researches on fossil microorganisms worldwide.
These researches involved not only stromatolites, but also other types of
sedimentary rocks such as Sandstones, Siltstones, Shale, and
carbonate rocks.
But what is the difference between stromatolites
the sedimentary rocks?
Stromatolites are rocks produced from sediments
of cyanobacteria, usually calcium carbonate. Gradually the original calcium
carbonate is solubilized by water and replaced with silica, which over time
becomes flint. Within the stromatolites are found microorganisms belonging to the same level of evolution:
prokaryotes, unicellular organisms where the chromosome is a long chain spread
throughout the cell.
Sedimentary rocks are produced by the
disintegration of rocks by weathering, transported and deposited by water in
the bottoms of lakes or seas. These rocks, during deposition, have imprisoned
within them the habitat of the waters in which they poured. Therefore, within
the sedimentary rocks are found fossils of microorganisms and organisms in different stages of evolution.
As we have seen the eukaryotic cell, from whose
evolution we also descend, begins through endosymbiosis, a prokaryote
swallowing another prokaryote. Probably at the same time a eukaryote acquired
by endosymbiosis a cyanobacterium, giving rise to a new eukaryotic cell
equipped with photosynthesis, called green algae, from whose evolution plants
descend. The eukaryotic cell is thus larger
than prokaryotes, and its chromosome is contained in a distinct central
nucleus. Incidentally, the theory of endosymbiosis, now widely accepted by
scientists, does not fit the Darwinian, more precisely Neodarwinian, view of evolution by mutation and
selection.
In ancient sedimentary rocks from 1.4 billion
years ago, in addition to several fossils of microorganisms that can be traced
back to prokaryotes, new fossils of microorganisms that do not resemble any
prokaryotes have been found.
J. W. Schopf, one of the leading experts on the
subject, had observed that the size of microfossils increased as geological age
decreased. Schopf later came to determine that prokaryotes did not exceed 10 µm
(micrometer) in diameter and only 2 specimens reached 60 µm.
Now, these new microfossils have an average
diameter of 250 µm and the most recent of 3000 µm. They, initially classified
as "Acritarchs" (of uncertain origin), were then identified as
ancient cells of eukaryotes that, through successive evolutionary stages,
became up to 10000 times larger than prokaryotes.
As Gonzalo Vidal reports, in "The First
Eukaryotic Cells," The Sciences 1984. The earliest eukaryote fossils were
found in the southern Urals and Montana and have been dated to about 1.4
billion years ago. Eukaryotes have been found in Argilloscists in Siltstones
and Sandstones at various locations around the planet of all ages from 1.4 to
0.6 billion years ago and had to be certainly microorganisms that lived freely
in the water, that is planktonic.
As usual, for the enormous temporal distance the
dates must be considered approximate.
As reported in “Precambrian Research” February
2019: Molecular clock estimates for the age of the last
eukaryotic common ancestor (LECA) range from c.
2300 Ma to c.1000 Ma and largely depend on the molecular clock
models and fossil calibrations used.
Recent findings of microfossils of eukaryotes in
the Changcheng Formation in northern China and the Mallapunyah Formation
northern Australia have been dated to 1,65 billion years ago. Schopf, moves the
presence of eukaryotes to 1.8 billion years ago. Fossils traceable to
eukaryotes have been found in rocks from that period in China, Russia, and
Ukraine. However, most finds of eukaryotic microfossils are found around
1.5-1.4 billion years ago. We can take the date of 1,5 billion years ago as the
certain date when there was an established presence of eukaryotes.
From the fossil record it seems ascertained that
autotrophic eukaryotes, the green algae, had a rapid spread soon after their
appearance, while heterotrophic eukaryotes, from which we also descend, had a
faster growth about 0.8 billion years ago.
The dependence of eukaryotes on a certain oxygen
content in the atmosphere and consequently a higher oxygen content in the
oceans seems also established.
One of the ways in which reproduction occurs in
eukaryotes is through mitosis. As Preston Cloud illustrates "La Biosfera",
Le Scienze 1983: "Mitosis depends on the contractile properties of actomyosin, which cannot form in the absence of
oxygen. Even the most advanced stages of the synthesis of sterols, fatty acids and
collagen, a fibrous protein whose appearance
leads to muscles and metazoans, depend on a sufficient level of oxygen".
We have already highlighted how 1.5 billion years
ago the content of oxygen in the atmosphere, initially 1/1000 of the current
reached, thanks to cyanobacteria, 1/100 of the current.
In conclusion, about 1,5 billion years ago, a
substantial increase of oxygen in the atmosphere, has allowed life, after 2
billion years since its appearance, to make a huge evolutionary step: the
appearance of eukaryotic cells, our ancestors.
As we pointed out in the previous article, four
fundamental events have produced ground-breaking breakthroughs that have
disrupted the course of events throughout the history of life. Without these
breakthroughs, life on our planet would have remained at the stage of
microorganisms. We have already mentioned the first: the aerobic photosynthesis
of cyanobacteria. Well, the second one is
the appearance of the eukaryotic cell because we are its descendants.
The development of eukaryotic cells had
alternating phases. As reported again by J. W. Schopf, around 1.1 billion years
there was a rapid growth of some genera of eukaryotes that, initially around a
diameter of 1.5 mm, began to grow in size to exceed, around 900 million years
ago, the diameter of 1 cm. It was during this period, according to Schopf, that
evolution, after having devised photosynthesis by cyanobacteria that caused a
revolution in the ecosystem for 2.5 billion years and gave rise to the
eukaryotic cell, devised a new revolutionary invention: sexuality.
Eukaryotes use two ways to multiply: mitosis,
which is a division of the same cell, a more complex cell division than in
prokaryotes, and meiosis, which is the fusion of two cells that mix their genetic material and subsequently divide giving rise
to a more rapid evolution. Schopf, in "La culla della vita" 2003,
suggests that, 1.1 billion years ago, the rapid growth of eukaryotes, may be
due to the invention of meiosis or, as is often said, sexuality. He highlights
how: «In an asexual population ten mutations
can produce eleven gene combinations (or genotypes-),
the starting one plus the ten of the new mutants. Instead in a population of
sexed organisms (assumed for simplicity genetically uniform, apart from the
pairs of genes where one is normal and one is mutant), the same ten mutations can
shuffle so as to produce 310 genotypes (about 60000)». And
further on, «The explosion of evolution, which began about 1.1
billion years ago, seems adequately explained by the advent of meiosis (the
division of sex cells) and the spread of eukaryotic sexuality, "the urge
to fuse." Meiosis, enhancement of the most primitive cell division of
mitotic type, was already at work at least around 950 million years ago,
approximately the age [...] of acritarchs provided with a wall with special
pores ("pilomata") for the emission of reproductive cells and small
marine algae (red algae) to sexual reproduction. Compared to asexual
eukaryotes, sexual eukaryotes must have evolved much faster, and were soon
enriched with many new types. Since such an explosion is found precisely in the
fossil record of the period between 1.1 billion and 900 million years ago, the
appearance of sexuality seems a sensible - and most likely correct -
explanation of this bursting beginning of evolution, the -ascent- phase in a
history of eukaryotes made of ups and downs».
It does not appear that this view has given rise
to any objections or opposing views.
In conclusion, 3.5 billion years ago appeared the
first innovative breakthrough that changed the course of events in the history
of life, the aerobic photosynthesis of cyanobacteria; about 1,5 billion years
ago appeared the second innovative breakthrough: eukaryotes, our ancestors; 1.1
billion years ago evolution produced the third fundamental breakthrough, sexuality, which defines the biochemical
structure of offspring.
At the date of 900 million years ago, just at the
height of their explosion, begins the decline of large eukaryotes until their
total disappearance, giving way to smaller eukaryotes.
But what caused their decline?
All bodies, at any temperature, emit heat in the
form of radiation, electromagnetic waves that fall in the invisible part of the
spectrum, the infrared. These waves, therefore, we do not see them but we can
perceive them, it is sufficient to approach sideways our hand to a hot body, for
example radiator. The heat that hits our hand are infrared waves. Planets are
hit by a wide range of electromagnetic waves from the sun, waves that fall
mainly in the visible part of the spectrum, absorb them and heat up. Some of
this heat is emitted into interplanetary space in the form of infrared waves.
As a function of the energy from the sun hitting the planets and the energy
emitted by the planets in the form of infrared, it is possible to calculate, in
an approximate way, the average temperature of the planet. This temperature can
then be compared with the temperatures actually observed through special
equipment. Thus, it turns out that:
Calculated temperature Observed temperature
Mercury 527° Kelvin (254° Celsius) 527° Kelvin (254° Celsius)
Mars 259° Kelvin (-19° Celsius) 250°
Kelvin (-23° Celsius)
Earth 246° Kelvin (-27° Celsius) 290° Kelvin (+17° Celsius)
(source:
Mario Ageno "Lezioni di Biofisica 2, 1984)
As you can see the earth has an observed
temperature about 50 degrees higher than calculated, because the presence of
the atmosphere causes a greenhouse effect.
Ultimately, the earth's atmosphere allows incoming rays to pass through, which
fall in the visible part of the spectrum. The heated earth emits waves that
fall in the infrared part and the atmosphere in part rejects them and sends
them back increasing the temperature of the planet. Technically, we say that
the atmosphere is transparent in the visible and not very transparent in the
infrared.
Earth's atmosphere is a mixture of gases, but the
ones responsible for the greenhouse effect are primarily water vapor and to a
much greater extent carbon dioxide or carbon dioxide, CO2. At an average
temperature of 17° Celsius a certain equilibrium is reached between the
incoming visible and the outgoing infrared, and the temperature remains
constant. But this temperature remains constant if the composition of the gases
composing the atmosphere remains constant, which is not at all obvious!
A global glaciation, referred to as Snowball Earth, that extended to near
the equator began about
right around the time of
the decline of the large eukaryotes and that, although with alternating phases
of long periods of glaciation and periods of rapid warming, lasted for 300
million years. Both Schopf (work cited above) and Robert M. Hazen's "Breve
storia della terra" 2017, agree that this glaciation was driven by a
climate change caused by two factors. On the one hand, a decrease in
atmospheric CO2 which, reacting with dissolved calcium in the oceans, has been
trapped in the form of limestone. At the same time there appears to have been a
strong growth of CO2-consuming cyanobacteria and green algae. Cyanobacteria and
green algae were subsequently buried by coastal erosion and then removed from
the recycling microorganisms that feed on the dead organisms they produce and
put back into circulation CO2.
According to Hazen, the strong growth of cyanobacteria and green algae was caused by the fracture, begun 900 million years ago, of the supercontinent Rodinia that at that time united all the landmasses.
This fracture gave rise, slowly, to new oceanic coasts soon
colonized by cyanobacteria and green algae. This fracture gave rise, slowly, to
new oceanic coasts soon colonized by cyanobacteria and green algae. These microorganisms,
consumers of CO2, increased dramatically. The strong reduction of CO2 in the
atmosphere has determined a thinning of the greenhouse effect, giving rise to
climate change. The atmosphere, which became more transparent to the infrared,
caused a drastic drop in temperature and resulted in a widening of the polar
ice caps. The white ice, more and more extended, reflected sunlight to space
accelerating the cooling of the planet. During the snowball earth phase, rather
than mass extinctions it was all life that was in danger of extinction.
Fortunately, the glaciers stopped near the equator. Microorganisms managed to
survive in a slice of the planet around the equator and near volcanic fumaroles
where temperate pools of water had been produced. The success of the
cyanobacteria was the cause of their near disappearance.
How did the earth recover from this global
winter?
Still according to Hazen it was volcanic gases,
the main component of which is CO2, that warmed the planet. Since the planet
was almost totally covered with ice, CO2 could not be removed as carbonate in
the oceans. In addition, the consumption of CO2, by the greatly diminished cyanobacteria,
was negligible. The concentration of this greenhouse gas in the atmosphere rose
rapidly to perhaps hundreds of times its current concentration, causing global
warming. It took the earth 300 million years to reach a new climatic
equilibrium, and it was this new equilibrium that produced the fourth
breakthrough. About 600 million years ago, without any consistent intermediate
fossil record, the fossil record, as if by magic, shows the presence of
multicellular organisms similar to small animals, our closest ancestors.
But that is yet another story. Giovanni Occhipinti
Next article end of february
Nessun commento:
Posta un commento