- What is life? Perhaps we’ll be able to give a better definition of it after we meet extraterrestrial life. But for now, I would settle for the idea that life is the cycle between birth and death. This cycle includes many functions, which we learn about in biology. However, life is so complex and so beautiful that it is correct to say that “life is a miracle”. It is a miracle that we encounter every day, and don’t value it, until a period comes that threatens to take it away from us. Then every morning I come out into the fresh air, think to myself “I am alive!”, start singing and feeling life: the wind, the temperature, the birds chirping, etc.
- If what has been said above about “life” is true, i.e. that it is the cycle between “birth” and “death”, then does it mean that stars are alive? How about other things that have a “life cycle” – does it mean that they are alive? The Universe was born in a “Big Bang” – does it mean the Universe is alive? What about the radioactive elements which have “half-life” period – does it mean they are alive? Either our definition of life is wrong, or the whole of the Universe is full of living things. Or perhaps there are different “levels” of life: 1) there is “life” of the Universe, 2) “life” of a galaxy, 3) “life” of a star, 4) “life” of the planetary system, which is a sub-system of a star, etc.
- In this section, we will try to discuss “life on Earth”, or rather its origin. Periods in the formation of life on Earth are shown in the following diagram. “Ma” stands for “millions of years”. Thus, first “hominids” (read: humans) emerged around 2 million years ago.
- The Miller-Urey experiment, conducted in 1952 (a nice illustration of which can be seen here), showed that complex organic compounds can arise from simple inorganic molecules in conditions resembling those of the primitive Earth. Hence, we attribute life on Earth to self-organization of matter. This theory is also called “Primordial soup theory“.
- A variation on the theory is the idea of “hydrothermal vents“, i.e. the idea that life began deep in the oceans near structures which are fissures “in a planet’s surface from which geothermally heated water issues”. These structures “are commonly found near volcanically active places, areas where tectonic plates are moving apart”.
- Still another theory of the origin of life on Earth is the “Panspermia theory“, i.e. the idea that life came to earth either in the form of microorganisms traveling on meteors, which bombarded early Earth, or it was sent to us from outer space on spaceships by other civilizations. However, if this theory is true, then it begs the same question: where did this “alien” life come from? The “Panspermia theory” appears similar to the idea of “god”: if god created the Universe, then who created god? And if it wasn’t created by anybody, then why the same reason cannot be applied to the Universe?
- Once primitive life forms formed, theory of evolution comes into play. The whole of Charles Darwin’s theory is stated most beautifully in the last paragraph of his book (“Origin of the Species”): “It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent upon each other in so complex a manner, have all been produced by laws acting around us. These laws, taken in the largest sense, being Growth with reproduction; Inheritance which is almost implied by reproduction; Variability from the indirect and direct action of the conditions of life, and from use and disuse; a Ratio of Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing Divergence of Character and the Extinction of less improved forms. Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone circling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved”.
- A nice illustration of development of life on Earth was given at a TED talk by David Christian. It is called “History of Our World in 18 Minutes”.
- Development of life can be divided into 4 geologic eras: 1) Precambrian (4.6 billion years ago – 542 million years ago). 2) Paleozoic era (542-250 million years ago). 3) Mesozoic era (250 million years ago – 65 million years ago). 4) Cenozoic era (65 million years ago – present). In the first, Precambrian period, the Earth cooled and life appeared in the oceans, for example as jellyfish. Oxygen was started to be accumulated in the atmosphere. In the second period, the Paleozoic, life started to move on land. The first massive extinction: “The Permian Extinction wiped out about 95% of marine life and nearly 70% of life on land.” In the third, Mesozoic era, dinosaurs became the dominant species, the “big bosses” of their times, but then were wiped out. It is now supposed that the reason for their extinction was a meteorite hitting the Earth. In the fourth, Cenozoic era, mammals became the dominant species on Earth. We experienced an “Ice Age”. And human civilization emerged (Source).
- How life developed on Earth is sketched nicely in Larry Gonick’s “The Cartoon History of the Universe”, vol. 1, “From Big Bang to evolution of humans as apes” (my notes are here). In particular:
In the oceans, life emerged from nutrient-rich “organic soup”.
Chemicals developed a genetic equipment and develop into cells.
Around 3 billion years ago, chlorophyll was developed. “When exposed to the sun, this green stuff enabled cells to get energy from the simplest food: carbon dioxide and water”. These became the first plants living in the water.
- The result of the plant’s energy production was oxygen. Oxygen reacted with other chemicals, such as metals, ammonia, methane and nitrogen, to produce cells which breathed oxygen and ate plants for energy. These became the first animals.
- For self-protection, both plan and animal cells clustered together in colonies, and hence produced first complex organisms consisting of many cells.
- Some cells began to specialize. Some became brains, others eyes, etc.
- Initially, life reproduced itself asexually, simply by cells dividing in two. Problem with asexual reproduction: there is no evolution. This is a problem in the survival struggle.
- Hence, cells developed sexual reproduction. Sex is good for developing different individuals, which is good for survival and development.
- Periods in development of life: 1. cambrian, 2. ordovician, 3. silurian, 4. devonian, 5. carboniferous, 6. permian, 7. triassic, 8. jurassic, 9. cretaceous, 10. tertiary, 11. quaternary.
- The cambrian period began with the first ice age. This caused an increase of competition for food, as a result of which some animals develop shells as a kind of armor, or protection.
- The ordovician period developed fish with cord running along their central nerve. This developed into a system of cartilage, which protected the central nervous system without any armor.
- The devonian period is known as “the age of fishes”. Fish develped brains, backbones, ribs, skulls, jaws, all of which allowed for a fierce struggle of survival. The big fish ate the smaller fish.
- The competition became so stiff, that some fish went on land, to escape the predators. But first on land were plants. This was in the ordovician and silurian periods.
- In the devonian period, plants covered the land, and animals followed. First came bugs, then fish.
- Fins gradually evolved into legs (the transition phase we see today in the seals).
- Carboneferous period is named for coal which was formed as a result of forests in this period
- Sexual habits kept the amphibians in water. To protect the future babies, a shell was developed around an egg.
- In the permian period, mammals developed solar collectors, for regulating body heat. These however were a problem when it was raining or very windy.
- Some animals learned to generate heat internally by burning food faster. This meant a need for more food. Hence, some animals developed a habit of walking on two legs, to run faster. Also, their teeth specialized into cutters and grinders.
- Hair was first used to extend the sense of touch. Later, it was used for protection and to keep warm.
- Triassic period – origin of dinosaurs.
- Jurassic period – small dinosaurs evolve into birds. Also, cousins of dinosaurs go into sea. In this period – around 200 million years ago – continental drift started.
- Cretaceous period – height of dinosaur period. Modern plants develop. 70 million years ago – sudden cooling. One theory – a giant asteroid hit the planet, as a result of which a giant cloud of dust was raised, and the climate cooled (see video here). Many kinds of sea life die. Dinosaurs die out in favor of small mammals. Apes are a kind of “primates”. We developed out of apes.
A study which is concerned with the development of the early life on Earth is “paleontology”. Definition which Wikipedia gives to paleontology is “the scientific study of life that existed prior to, and sometimes including, the start of the Holocene Epoch (roughly 11,700 years before present)”. Using special techniques, paleontologists “discover much of the evolutionary history of life, almost all the way back to when Earth became capable of supporting life, about 3.8 billion years ago”. It seems that “Understanding Evolution: History, Theory, Evidence, and Implications”, by R.G. Price, 2006 is a good (secondary) book on evolution, as it covers the history of the problem – myths and beliefs about origin of life which existed in various cultures prior to present – and the current theory on the subject.
Important personalities for understanding the origin of life on Earth are: 1) Charles Darwin, 1809-1882. I recommend his short “Autobiography”. 2) Gregor Mendel, 1822-1884. Mendel’s pea plant experiments conducted between 1856 and 1863 established many of the rules of heredity. 3) Francis Crick, 1916-2004, and James Watson, b. 1928 – discovered the structure of DNA in 1953.
Summary: if life is a cycle between birth and death, then everything is alive, starting with our Universe and the Sun. Life cycle is specific to the kind of thing we’re discussing. Thus, life cycle of a star has one pattern, while life cycle of a cell has another pattern.