If life were predictable it would cease to be life.
Eleanor Roosevelt (1884-1962).
I would rather have a miserable ape for a grandfather than a man
[who employs his great]
faculties and influence for the mere
purpose of introducing ridicule into a grave scientific discussion. Thomas H. Huxley
(defending Darwin
against the bishop of Oxford)
(2002-11-11)
Evolution of Life
How life begets new life (following the enigmatic
apparition of Life)?
The basic mechanisms of the evolution of species are now well understood.
Normally, the so-called gene pool of a given species is larger than
the number of genes of a given member of the species.
If one gene gives a particular advantage to the survival of its bearer,
it will tend to be more frequent among individuals that have survived long
enough to reproduce. The frequency of such a gene will therefore increase from
one generation to the next until most members of a species carry it.
This is the idea of
natural selection first advocated
by Charles Darwin
(1809-1882),
whereas the broader idea of the evolution of species is a much older concept
(known to the ancient Greeks).
If a sudden change in the environment makes a gene essential to individual survival,
it may even be the case that all members of a new generation will end up
carrying the life-saving gene.
This is what happens among insects when a new insecticide is massively introduced:
The so-called susceptible insects are eradicated but there may be a few
resistant individuals whose descendants will multiply at an increased rate
to fill the space vacated by a much higher mortality rate among the rest of the species.
The richness of the gene pool is thus essential for species to respond to new threats.
This richness is depleted when a gene is "used" in response to an actual attack
(as the "better" gene completely replaces alternate choices) but the gene pool
is also replenished by random mutations, which occur at a very slow rate
due to rare errors in genetic duplications
(which may be induced by cosmic rays, or other violent causes at the molecular level).
Even in the absence of drastic environmental changes, the above evolutionary mechanism
allows species to become better adapted and/or more competitive with other species
in the same niche.
Some species keep winning the survival race for millions of years, others become extinct
much faster.
Quite a few drastic "inventions" have occurred in the evolution
of life on this planet of ours.
Some of these are so radical that it may be hard to rule out the existence
of something more potent than the above evolutionary process.
For now, however, we may have no other solution but to
believe (rightly or wrongly) that, given enough time, unlikely events do occur
and may help generate previously unknown innovations.
First among these mysterious innovations is the appearance of life itself from a
prebiotic environment.
We discuss this in the next article;
a few other deep life mysteries are listed below:
Homochirality
All biological molecules are skewed in only one of two possible ways.
Nucleotides are righthanded, amino acids are lefthanded. Although artificial synthesis can produce
both handedness in equal proportions, only one form of glucose (dextrose)
is produced or consumed by living organisms.
The physical laws that are relevant to chemistry do not distinguish between left and right
(only the weak nuclear force does that and it seems completely unable to influence
the chirality of chemical reactions).
However, the dominant chirality of food would so potently favor the lifeforms can consume it
that other types of lifeforms would soon be eradicated (if they had managed to survive and
evolve in a separate habitat).
Nowadays, life exists on Earth in only one type of handedness.
This is called homochirality.
A lone organism from outer space endowed with the wrong chirality could
not survive on Earth (except in suspended animation)
for total lack of food...
Likewise,
there are many examples
of physical systems that evolve from symmetrical conditions into
a completely antisymmetrical situation.
Proteins Synthesis
Nucleus
Sex
Symbiosis and Endosymbiosis
Lichens are, in fact, the mutually beneficial association of two organisms a
fungus (called mycobiont) and a cyanobacteria
(called photobiont).
The fungus provides minerals, moisture and protection from overexposure to sunlight,
whereas the photobiont is able to photosynthesize organic food from the
carbon dioxide in the air, even when no other food supply is available.
This type of association is known as a symbiosis;
it allows at least one of the constituents to live under conditions where it could
not survive by itself.
Not all types of symbioses are mutually beneficial, but most of them are.
Note that a lichen's photobiont is often described as an alga
in most introductory presentations,
which are clearly unduly influenced by the fact that
cyanobacteria are still called
"blue-green algae" in spite of the fact that these
photosynthetic prokaryotes are not algae at all...
There is an even closer form of symbiosis,
called endosymbiosis, which is exemplified by all higher
life forms, including you and me.
Each of our own cells harbors a number of mitochondria, which are
(like blue-green algae) an elementary form of bacteria lacking a nucleus.
Human cells could not function without them,
and human mitochondria only reproduce within a human cell...
However, mitochondria have their own genetic material (a single chromosome
containing a circular strand of DNA) and their replication is independent from the
replication of the rest of the human cell which harbors them
When a human cell divides, after replicating its nucleus, about half of the mitochondria
in it will go into each new cell and they will reproduce there, as needed.
As human tissue grows, the mitochondrial population within its cells grows as well.
When a human egg is fertilized by sperm, it so happens that
the male mitochondria remain in the sperm's tail, which never enters the egg.
Therefore, all mitochondria in a human embryo will be the same as
the mitochondria present in the cells of the mother.
Mitochondrial DNA (often abbreviated mtDNA)
is thus inherited from mother to daughter in an asexual way.
As the evolution of mtDNA is only subject to slow mutations over time,
it has been shown that all human maternal lineages have died out, except one...
Therefore, all humans are descendants of a single woman
(called the "mitochondrial Eve") who lived about 200 000 years ago.
She probably lived in Africa, because the mtDNA of African populations shows
more variation than what is observed on other continents:
This suggests that the descendants of "Eve" lived exclusively in Africa for a long
period of time before migrating to other regions.
This is the basis of what's now known as the "Out of Africa" theory.
Neanderthals became extinct (and/or were exterminated by our Cro-Magnon ancestors)
about 30 000 years ago.
Mitochondrial DNA from their bones showed that their species was clearly separate from
our own. They are not our ancestors,
contrary to what was previously thought, and we rarely interbred.
(2023-06-13) Human have 46 chromosomes. Other great apes have 48.
Human chromosome pair #2 (HSA2) looks like the fusion of two ancestral chromosomes found in chimpanzees (PTR12 and PTR13).
The fact that the normal karyotype of humans contains 46 chromosomes was cautiously presented in 1956.
At the time, chromosomal mosaicism (variations of the number of chromosomes from one cell to the next within
the same organism) was thought to be more prevalent than it actually is.
So, the authors of the epoch-making 1956 were reluctant to generalize the observations
they had made on relatively few cells (without a single counterexample, though).
All extant non-human great apes have 48 chromosomes instead of the 46 in the human karyotype.
Among lesser apes, siamangs have 50 chromosomes
and other gibbons typically have 44 chromosomes.
The venomous Jack Jumper ant from Ausralia
(Myrmecia pilosula) has a single pair of chromosomes.
The picture at left is from a 2016 article by the geneticist
Pawel Stankiewicz,
published in Molecular Cytogenetics One pedigree we all may have come from. Did Adam and Eve have the chromosome 2 fusion?
Volume 9, Article number: 72 (2016
This picture demonstrates that the second chromosome of Homo Sapiens (HSA2) has
bands matching those of two lesser chromosomes PTR12 and PTR13 from modern chimpanzees
(PTR = Pan TRoglodyte). The HSA2 results
from the fusion of two ancestral chromosomes is confirmed by the presence
in the center of HSA2, od two DNA sequences normally found only at the ends of chromosomes.
Robinsonian fusions like the one which resulted in the human chrmosome HSA2 destroy few genes, if any,
and typically won't affect the health or the fertility of the affected individual.
When the fused chromosome is passed on to an offspring, the karyotype of the offspring may
simply have a missing pair.
If this chromosomal rearragement happens to present an evolutionary advantage, the change
may become dominant after a few generations, especially in a polygamous and incestuous tribe.
This may well be the origin of the split between the respective lineages of
chimpanzees and humans, about 5 million years ago.
The different numbers of chromosomes would instantly have created a reproductive barrier.
If that's the case, all humans would descend from a single individual (most likely male
in a polygamous and/or incestuous society) living at the time of the final split from apes.
Although
creationists dispute this,
their account of the history of the topic (which emerged in 1982)
is a valid contribution of theirs.
This fusion event is thought to have occurred only once.
Estimates vary widely, but some philogenetiic arguments
would trace this event back to a single individual who lived about 5 million years ago.
I propose to call this individual Lot
after the biblical character who mated with his two daughters on consecutive nights to prevent the extinction
of his family, after the anhilation of
Sodom and Gomorrah,
including Lot's wife.
Our lineage split from that of gorillas 7 to 10 million years ago. All the aforementioned
African apes separated from the Indonesian lineage of modern
orangutans 12 to 16 million years ago.
All apes split from (tailed) monkeys around 25 to 30 million years ago.
Among monkeys, chromosome numbers can range from 40 to 62, but it seems that only apes have 46 or 48
(only modern humans and their direct ancestors have 46).
Monkeys and apes come from the early primates which started appearing after
the abrupt extinction of large dinosaurs, 65 millions years ago.
The oldest known hominid species is
Sahelanthropus tchadensis,
dating back approximately 6 to 7 million years. around the time of
the split between our lineage and that of
the two panins
(Bonobo and
chimpanzees, our closest living relatives)
Fossil remains of Sahelanthropus tchadensis were discovered in Chad, Africa,
specifically in the Toros-Menalla region, during expeditions conducted between 2001 and 2002.
The most notable fossil specimen, nicknamed "Toumaï," consists of a partial cranium and a few other fragments.
Sahelanthropus tchadensis is significant because it represents one of the earliest potential ancestors of the human lineage.
Although the available fossil evidence is limited, it exhibits a mix of primitive and more derived features,
suggesting some adaptations towards bipedalism.
However, the exact placement of Sahelanthropus within the hominid family tree and its relationship to other early hominids
are still subjects of scientific debate and ongoing research.
Other early hominid species that lived around 4 to 5 million years ago include
Ardipithecus ramidus and
Orrorin tugenensis,
both of which have contributed to our understanding of human evolutionary history.
These early hominids provide insights into the origins of bipedal locomotion and the diversification of the human lineage,
which is trditonally presented in the following chronological order of appearance:
Ardipithecus ramidus lived approximately from 6 to 4 million
years ago. They are the most ape-like fossil hominin to date, showing adaptation to both bipdalism
(walking on two legs) and life in trees.
Ardipitecus ramidus is considered to have been more closely related to humans than chimpanzees, at a time when the two lineages split.
Australopithecus anamensis.
They lived approximately from 4.4 to 3.8 million years ago. The most famous and most ancient specimen
is Ardi (ARA-VP-6/500). They were not as efficient at bipedality as humans,
and not as adapted to arboreality as non-human great apes.
They seem be be the direct ancestors of Australopithecus afarensis, with whom they have coexisted.
Australopithecus afarensis:
This species, which lived in the pliocene approximately 3.9 to 2.9 million years ago,
is famous for the well-preserved fossil called "Lucy"
(AL 288-1) discovered in 1973-74 in the Afar region of Ethiopia.
Australopithecus afarensis is considered an early hominin and is known for its bipedal locomotion.
Lucy was a young adult at the time of her death but she stood only 3.7 feet (1.10 m). She suffered from
flat feet and
genu valgum brought about by a Vitamin-D deficieny
due to insufficient exposure to direct sunlight, which may indicate that she had spent too much of her life confined to a cave.
Homo habilis (sometimes still called Australopithecus habilis):
This species, dating back around 2.4 to 1.4 million years ago, is one of the earliest members of the genus Homo.
Homo habilis is known for its use of stone tools and is considered to be more closely related to humans than the Australopithecus species.
Homo erectus: Homo erectus is a major extinct hominid species
that existed approximately from 1.8 million to 100,000 years ago.
They were the first hominids to walk completely upright. They mastered fire, cooked their food and invented spears.
They are the most successful human species ever, both in terms of temporal span and geographical spread.
They had a larger brain size and a more sophisticated tool-making ability compared to earlier hominids.
They probably invented the Knappedhand axe (French: biface)
made from chert
(flintstone; French: silex)
the longest-used tool in human history.
Homo erectus is believed to be the first hominid to have expanded beyond Africa.$nbsp;
So far, we only have DNA fragments from Homo Erectus, which are not sufficient to sequence a complete genome,
but this may change soon due to the existence of fairly recent remains.
Homo ergaster: Homo ergaster is an hominid species
that arose between 1.9 and 1.7 million years ago and became extinct about 1.4 million years ago.
They may have arisen before homo erectus but died out much sooner
and are sometimes not considered ancestral to modern humans. However, the data isn't entirely clear and
the issue is still debated. Some have described homo ergaster as a subspecies of homo erectus, which
contradicts the timeline presented here.
Homo Antecessor: Homo Antecessor
is an extinct hominid species that lived approximately 1.2 to 0.77 million years ago, during the Early Pleistocene.
It is believed to be an ancestor of Homo Heidelbergensis, Neanderthals and modern humans.
Homo Heidelbergensis: Homo heidelbergensis
is an hominid species that lived approximately 700,000 to 200,000 years ago.
It is believed to be an ancestor of both Neanderthals and modern humans, possibly the last one.
Fossil evidence suggests that Homo heidelbergensis had a larger brain size than earlier hominids
and exhibited more advanced tool-making abilities.
Neanderthals (Homo neanderthalensis):
Neanderthals lived in Europe and parts of Asia
from approximately 450,000 to 40,000 years ago.
They had a robust build and are known for their distinct cultural behaviors, including the use of tools,
symbolic expression, and burying their dead.
During the time of the Mousterian culture, neanderthals introduced the craft of
hafting handles from wood or bones to stone bifaces
to produce something resemblig axes, javelins or spears.%nbsp;
Neanderthals are not ancestors of modern humans but there
was some interbreeding at times,
until about 65,000 years ago. Interbreeding was more common between Neanderthal males and modern human females.
The entire genome of a 430,000-year-old specimen has been sequenced, using thigh-bone fragment discovered at the
Sima de los Huesos ("Pit of Bones") in Spain,
which was [reviously misidentified as Homo heidelbergensis (see above) until 2014. This helped revise the
hypotheticals age of the earliest neanderthals from 400,000 years to 450,000 at least.
Denisovans: Denisovans are an extinct group of hominins
known primarily from DNA analysis of a finger bone and a few teeth found in the
Denisova Cave of Siberia.
They are believed to have lived from around 400,000 to 40,000 years ago.
Genetic studies have revealed that Denisovans interbred with both Neanderthals and early modern humans.
In 2012, a 2 cm bone fragment was found in Denisova cave belonging to a 13-year-old girl nicknamed
Denny (Denisova 11)
who lived about 90,000 years ago and is the only known example of a first-generation
hybrid hominin, She had a pure Neanderthal mother and a Denisovan father, who himself had a minuscule
Neanderthal ancestry (dating back to a few hundred generations prior).
Homo naledi: Homo naledi is
a recently discovered (2013) hominin species found in the
Rising Star Cave system of South Africa.
They are estimated to have lived between 335,000 and 236,000 years ago. The discovery is significant because
the remains exhibit a mix of primitive and more modern human-like traits.
Homo sapiens: Modern humans. Homo sapiens
emerged around 300,000 years ago in Africa
and are the only extant species of the genus Homo. That's all of us.
One of their early inventions was the bow-and-arrow weapon,
which could efficiently kill at a distance.
This may have been instrumental in the extermination of their neanderthal rivals, which they couldn't
quite match in close combat. Possibly the first deliberate genocide brought about by technology.
The war of attrition
between Cro-magnons
and Neanderthals lasted thousands of years. The oldest known arrowheads were found in 2010 in the
Sibudu Cave of South Africa. estimated to be around 64,000 years old, they were carefully crafted from bone
and featured barbs and serrated edges.
Homo floresiensis, often referred to as "Hobbits" because of their small stature.
They ived in the Indonesian island of Flores approximately 100,000 to 60,000 years ago.
The date they actually appeared is currently debated.
They had a unique combination of primitive and advanced features and
were significantly smaller in body and brain size compared to modern humans.
The latest discovery in paleoanthropology (2019) is Homo luzonensis,
on the basis of fossil remains found in Callao Cave in the Philippines.
Homo luzonensis is estimated to have lived around 67,000 to 50,000 years ago.
The discovery includes teeth, finger and toe bones, and a few other fragments,
providing evidence of a previously unknown hominid species that coexisted with
other hominins in Southeast Asia during that time period.
(2003-01-03) Fossil Calendars
Over long periods, calendrical ratios do change.
Modern nautilus shells
invariably show about
30 daily growth lines
between their chamber partitions, called septa, whose development is
synchronized with the actual lunar month (currently about 29.5305889 days).
Nautiloids first appeared about 420 million years ago, when the solar day was
about 21 hours [1 hour = 3600 atomicSI seconds].
The fossil record shows that the earliest nautiloids had
only 9 growth lines between septa:
420 million years ago, there were about 9 days (of 21 hours) in a lunar month !
The distance to the Moon was only 40% of what it is today,
so the apparent diameter of the Moon was about 2½ times what it is now.
Total solar eclipses were more common than partial eclipses today.
Even the rate
of recession of the Moon
does not remain constant over the ages.
The strength of tidal effects is strongly dependent on the configuration of the
continents (and/or the ocean floor) which is extremely variable over geological
time periods.
Currently, the Moon recesses from the Earth at the comparatively rapid rate
of 38.2(7) mm per year (Dickey et al., 1994).
The paleontological study of so-called tidally laminated sediments
(also called tidal rhythmites)
has shown conclusively that this recession speed has varied greatly,
but it was typically much slower in the distant past.
If this wasn't so, the Earth-Moon system couldn't have formed at the
time indicated by radioactive dating (about 4½ billion years ago).
Some
models
explain the formation of the Earth-Moon system by a collision of
the young Earth with an object 10 times smaller than itself.
A weaker tidal braking in the past would seem like a paradox at first,
since a closer Moon should have produced stronger tides.
However, this general trend could be more than compensated by the large differences
in the heights of the tides around different configurations of land masses.
This effect is commonly observed when comparing different coastlines,
and it would dominate globally as the continents drift.
Everything seems to indicate that tidal effects are currently way above average,
.>......so the current rate of recession is a poor indication of what happened in the distant past.
(2020-08-18) Last Universal Common Ancestor (LUCA)
Latest lifeform among the ancestors of all extant species.
The RNA world became extinct more than 3 billion years ago but all
modern lifeforms on Earth come from it. Anytimr a rare species become extinct,
the LUCA title may be passed on to a more recent creature.
LUCA never gets older.
It would be interersibg to plot the age of LUCA as time passes.
The slope of that curve is a dimensionless number (year/year)
and can be expressed as a percentage expressing the decay of biodiversity
in a natural long-term way (more meaningful than the mere
number of species disappearing each year).
Unfortunately, this is not yet a popular metric.
That would be a worthy topic for a doctoral dissertation in paleontology...
That fundamental parameter doesn't depend on the modern diversification of species nor
on the extinction of recent variations.
Life probably all began in deep-sea hydrothermal vents. LUCA may be that ancient.
Its currently estimated to have lived at a specific date beweeen 3.5 and 3.8 billion years ago.
The approximate date when each interval begins is shown in the first column, in
millions of years (Ma) before the present time. 0.011 Mya = 11,000 years ago.
(2003-01-03) Fossil Calendars
