Emergence of life

Life evolved from a simple cell to a complex organism. It is known as the evolution theory, which Darwin proposed. The foundation of the evolution theory is laid on the underlying techniques that allow us to go back in time. For example, radiometric dating is one technique used to estimate the age of fossils. Here, we discuss a few methods used for dating fossils.

Fossils Dating

A few methods have been developed to find the age of an object based on scientific observations of the process of decaying elements. One of the techniques, radiometric dating, is well-established and provides results with known accuracy. It is based on the fact that some basic elements occur in two or more atomic forms known as isotopes. The isotopes have the same number of protons in the nucleus of an atom but differ in the total number of neutrons. The chemical element carbon is one of the main constituents of all living beings. Therefore, carbon is one of the popular elements used in estimating the period of living beings, and the method is called carbon dating. Carbon has two isotopes which are denoted C12 and C14. The isotope C14 decays with time by emitting radiation and is known to be radioactive. All living beings consume C14 along with C12 while alive. Thus, they have the same concentration of C14 inside the body as C14 in the atmosphere. However, after the death of a living entity, the consumption of C14 stops. The internal C14 starts to decay with time at the rate determined by the laws of radioactive decay. The decay rate has been measured in the laboratories and is constant. In the carbon dating method, a sample of the dead body is used. The amount of C14 remaining after decay in the sample is measured. From the known decay rate, the time elapsed since death is computed.

The molecular clock is another technique used for dating fossils. It is based on the analysis of DNA and its mutation rate. Molecular clock dating is based on the observation that significant evolutionary changes occur over specific time intervals. The molecular clock provides additional support for carbon dating. The use of dating techniques and circumstantial evidence makes it possible to get an excellent estimate of the period of the life of various species from the fossils.

Timeline for the origin of life

We mentioned that hydrogen and helium were the first two elements formed at the beginning of the universe 13.8 billion years ago. Dense clouds of hydrogen and helium started creating the stars about 400 million years after the start of the universe.

The collapse of hydrogen and helium atoms due to gravity starts a fusion reaction in the core of the stars. The fusion of hydrogen and helium atoms makes heavier atoms, such as carbon and oxygen. Nitrogen, phosphorus, iron, etc., are produced in the ongoing fusion reaction over a long time. During its active lifetime, a star keeps on consuming hydrogen. At the end of its life, hydrogen gets depleted, and a star dies in a supernova explosion. The explosion spreads all its atoms which nearby stars can gather. An estimate shows that our star, the Sun, was formed more than 9 billion years after the universe began. Earth was born as its planet in the planetary disc around the young Sun. Thus, there is a possibility that Earth's essential life-forming elements came from other stars.

With the cooling of early Earth, hydrogen and oxygen atoms combine to make water molecules. Also, combining carbon, hydrogen, and oxygen atoms produce sugar molecules. Simple molecules such as water and sugar combined further with nitrogen and phosphorus to give rise to primary molecules of nucleic acids. From the molecules of nucleic acids, RNA formation led to a simple single-cell organism. On early Earth, several types of single-cell organisms evolved. All the forms of life existing on Earth are broadly classified into three categories. These categories are the three lineages of life known as the eukaryote, the bacteria, and the archaea. Bacteria are the smallest living cells. A base cell of bacteria is classified as a prokaryote type, while all the plants and animal cells are classified as a eukaryote type.

 Various prokaryote bacteria cells were the first to evolve on early Earth. Therefore, some theories assume that the prokaryote cell gave rise to other cells. One such theory is known as the endosymbiotic theory.

The evolutionary endosymbiotic theory suggests that the eukaryote cells have evolved from the prokaryote cells. According to this theory, a growing prokaryote cell engulfed another cell. The engulfed prokaryote bacteria cell existed in two versions. In the case of an engulfed oxygen-breathing bacteria, it gave rise to an animal cell. While for an engulfed photosynthetic bacterium, it gave rise to the plant cell. Thus, the growing prokaryote cells resulted in two different eukaryote cells. These two eukaryote cells have evolved into animal and plant cells, respectively, as shown in Figure 2.7.

As mentioned earlier, most of the characteristics of a living being are determined by the genes in DNA. The genes can express different traits in plants or animals.

Figure 2.7: An illustration depicting the endosymbiotic theory.

Credit: https://philschatz.com/biology-concepts-book/contents/m45513.html [https://creativecommons.org/licenses/by/4.0/].

 

The DNA analysis of the three lineages of life shows that they share a common ancestor termed the last universal common ancestor (LUCA). Therefore, it acts as evidence that the origin of all forms of life has a common root. Furthermore, scientists have identified more than 300 common genes that were the starting point for all life forms. Therefore, it is possible to classify information on biodiversity in the form of the Tree of Life, as shown in Figure 2.8. The figure depicts the evolution of life on Earth originating from a common root (LUCA).

The Tree of Life is based on the analysis of the common ancestors of species. For example, on the Tree of Life, humans are shown at the top right corner indicating that humans evolved after a long line of progenitors. Also, it shows that bacteria were the earliest microorganisms.

Figure 2.8: Illustration depicting the evolution of life, giving rise to the three domains of life; Archaea, Bacteria, and Eukaryote.

Credit: From an article available at NASA Astrobiology Institute website, [Public domain], modified by the author.

 

A timeline for the evolution of life on Earth is presented in Figure 2.9.

The analysis shows that Earth formed about 4.54 billion years ago. However, it took about half a billion years for environmental conditions to become favorable for life. The earliest evidence of life on Earth is preserved in the rock fossils, which serve as the manifested record of life. For example, in Western Australia, microorganisms similar to bacteria have been found fossilized in the sedimentary rocks indicating that biotic life started about 4.1 billion years ago.

Figure 2.9: Graphic illustration of the timeline for life evolution on Earth.

 

As shown in the figure, a primary prokaryote cell that gave rise to life is estimated to be 3.8 billion years old. Likewise, the earliest evidence of photosynthesis, a process of using sunlight to synthesize life-sustaining food using carbon dioxide in plants, is found to be 3.4 billion years old.

The formation of complex cells leading to the life of the eukaryotes happened about 2 billion years ago. These complex cells took about 500 million years to build multi-cellular life. Finally, these multi-cell entities evolved into simple animals.

About 541 million years ago, there was a rapid growth in the variety of species that emerged quickly. This rapid growth in the number of species is known as the Cambrian explosion. A category of organisms that tend to mate with each other is identified as species. Further, species originating from a common ancestor form a branch known as the 'genus.' Anatomical modern humans are on the terminal twig of the Tree of Life, as shown on the right top branch of animals in Figure 2.8. This twig is nested with the great apes within the primate branch. The primates belong to the mammals within the vertebrates. The vertebrate, in turn, branches out from the metazoans within the eukaryotes.