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The Precambrian time is divided into two parts; the older Archean Eon and the younger Proterozoic Eon. This period is marked by significant changes in the atmosphere and the oceans. The Proterozoic rocks which have been identified contain fossil traces of the primitive life-forms of blue-green algae and bacteria (Young, 2017). Additionally, there are fossils of Ediacaran fauna, the first oxygen-dependent organisms. Before life arose on earth, its atmosphere was mainly filled with carbon dioxide and nitrogen and small amounts of hydrogen, hydrogen sulfide, and carbon II oxide; the oxygen levels were very little.
The limited atmospheric conditions enabled organic molecules to form spontaneously in the presence of an energy source, such as sunlight. The organic molecules then polymerized impulsively under conceivable prebiotic environments to form macromolecules. Among the macromolecules, there were the nucleic acids, which could self-replicate. An example is the RNA from which the first cell is thought to have risen; it is replicated in an enclosure of a membrane made of phospholipids. These led to the formation of prokaryotes, which are unicellular organisms.
The cyanobacteria were the first primitive protists to demonstrate photoreceptive. The prokaryotes were unable to satisfy their energy requirements, and they developed various mechanisms to generate energy in the form of adenosine triphosphate (ATP). These mechanisms evolved in three stages, which corresponded to glycolysis, oxidative metabolism, and photosynthesis. The latter played a key role in evolution as oxygen was one of the end products, besides energy, and the process mainly occurred in cyanobacteria (Hörandl and Hadacek, 2020). The oxygen levels in the atmosphere increased significantly, and it was around this time that the first eukaryotes appeared on earth.
The first eukaryotes were single-celled like prokaryotes, and a good example is a yeast. Multicellular organisms then evolved from these eukaryotes more than 1.7 billion years ago (Hörandl and Hadacek, 2020). Additionally, some of the single cells of eukaryotes, such as the green alga, formed aggregates, representing the evolutionary transition from unicellular to multicellular organisms. Such colonies of cells are presumed to be the precursors of planets.
There was increased specialization of cells, which resulted in a complete transition to truly multicellular beings. Complexity and diversity followed the specialization, leading to present-day animals and plants. According to Pennisi (2018), the first multicellular animal was the Grypania spiralis. Its coil-shaped fossils were found in the United States of America, South Africa, and Asia.
The first bilateral animals were found within the Ediacaran Biota. The Ediacaran taxa are rare, and they are referred to as Kimberella (Evans et al., 2020). The energy from sunlight used in photosynthesis existed between the ultraviolet and infrared wavelengths. As the cyanobacteria utilized the emissions, there was a production of beta carotenes, essential elements in the formation of retinal. The opsin evolved from G protein-coupled receptors and combined with retinal that served as a proton pump. This formed the basis of the metazoan photoreceptive elements such as ciliary opsins, photoisomerases, and rhabdomeric opsins (Schwab, 2018). Other photoreceptive substances such as chlorophyll, pili proteins, and flavins could not compete successfully with rhodopsin to be the principal photoreceptors.
The opsin and retinal bonded in an eyespot, and through evolution, the organisms used it for sight instead of a proton pump. By then, most cells with the eyespot would identify only light and dark. It took over 35,000 generations for organisms to discover that a concave cup instead of a spot would provide effective sight and hence produce a more competitive organism (Schwab, 2018). In pigmented unicellular organisms such as dinoflagellates, they have a unique eye-like structure referred to as an ocelloid (Nilsson and Marshall, 2020). Its lens refracts light such that it passes through the photoreceptor structure. The development of a fully developed camera-style eye occurred through 364,000 generations (Schwab, 2018). Later on, in the eye evolution, the crystalline lens was added to the eyes. This includes the lens, cornea, ocular adnexa, and extraocular muscles.
In addition to the camera-style eye, a compound eye also evolved from the eyespot. During its evolution, the light-sensing lens bulged or formed a convex protrusion from the cellular layer. Hence, the characteristic round lens-like discs on the outside while the inner aspects lack convexities (Schoenemann, Pärnaste, and Clarkson, 2017). This helped the organisms obtain spatial information, and hence, it was referred to as an eye.
Another lens was added, and later on, a second lens was placed proximally to the first, creating a unit of the compound eye, referred to as an ommatidium (Schwab, 2018). With time, multiple ommatidia were formed through gene duplication to form several units. The compound eyes can be seen in organisms such as the dragonfly. Eventually, the morphology of the compound eyes evolved, and it formed over six categories. The groupings include reflecting superposition, neural superposition, parabolic superposition, refracting superposition, afocal apposition, and apposition. Each class has varying optics and neurologic channels that are beneficial to their niches.
It is important to note that the eyespot and photoreceptors have the same absorption spectrum. On the one hand, the photoreceptive compounds were composed of retinal and opsins, and they were sufficient to transduce light to produce the energy required for biochemical activities (Schwab, 2018). On the other hand, the eyespot was formed via the covalent bonding of opsin and retinal, and through evolution, it was used for the sign. Therefore, the eyespot and the photoreceptors are made of the same elements; hence, they have the same absorption spectrum.
Reference List
Evans, S. D. et al. (2020) ‘Discovery of the oldest bilaterian from the Ediacaran of South Australia,’ Proceedings of the National Academy of Sciences, 117(14), pp. 7845-7850. Web.
Hörandl, E. and Hadacek, F. (2020) ‘Oxygen, life forms, and the evolution of sexes in multicellular eukaryotes,’ Heredity, 125 (1), pp. 1-14. Web.
Nilsson, D. E. and Marshall, J. (2020) ‘Lens eyes in protists,’ Current Biology, 30(10), pp. R458-R459. Web.
Pennisi, E. (2018) ‘The momentous transition to multicellular life may not have been so hard after all,’ Science. Web.
Schoenemann, B., Pärnaste, H. and Clarkson, E.N. (2017) ‘Structure and function of a compound eye, more than half a billion years old,’ Proceedings of the National Academy of Sciences, 114(51), pp. 13489-13494. Web.
Schwab, I.R. (2018) ‘The evolution of eyes: Major steps. The Keeler lecture 2017: Centenary of Keeler Ltd’, Eye, 32(2), pp. 302-313. Web.
Young, G. M. (2017) ‘Precambrian: Overview. Elsevier’, in Earth Systems and Environmental Sciences. Elsevier. Web.
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