For 3 billion years, life on Earth merely comprised of single-celled microbes. It was only after the appearance of photosynthesis and oxygen in atmosphere that multicellular and sexual organisms came to dominate, a billion years ago. The fact that the jump from single-celled to multi-celled microbes took 3 billion years is a testament to the stability of unicellular ecosystems, as well as the marked complexity of even the simplest multicellular creatures.
Multicellular’s main evolutionary benefit is having its own little ecosystem, complete with multiple types of unicellular tissues, within an enclosed bodily boundary. As discussed, an ecosystem of competitive and collaborative microorganisms had already proven to be stable and robust in dealing with environmental challenges, over the first 3 billion years of life. Similarly, once established, the mini-ecosystem within the multicellular organism proved even more advantageous for survival through the struggles of natural-selection.
However, to reproduce, the multicellular organism had to overcome the very complex barrier of regenerating a whole organism (with different cell types) from two half-gene germ cells (sperm and egg). Sexual reproduction required at least 5 distinct features that were absent in the simple unicellular duplication, where each mother single-cell only had to divide into two identical daughter single-cells. Namely: 1- The multicellular organism had to produce sexual cells with half the genes of its whole cells. 2- The sperm half-cell had to fertilize the egg half-cell. 3- The fertilized egg (zygote) had to divide into multiple cells with identical genes (embryo). 4- The embryo cells had to go through differentiations that activated different sets of their identical genes. 5- The differentiated embryo cells had to further divide and duplicate to produce all the different types of cells in the multicellular body.
The first multicellular creatures (algae and fungi) were rather simple and had no more than 10 different cells in their body. In comparison, plants have 20-30 different cells and the more complex animals have evolved to have 100 to 150 different cells in our bodies.
But what is the evolutionary advantage of sexual reproduction? Why have such a great number of multicellular organisms emerged and were naturally selected after the sexual revolution of 1 billion years ago? The answer lies in the great genetic variety that sexual reproduction can generate, compared to the simple copy-and-paste of asexual doubling of single-celled microbes.
With asexual organisms, genes are copied and inherited completely during reproduction through duplication. In contrast, the offspring of sexual organisms contain random mixtures of their parents' genes, which are produced through independent assortment (shuffling of genetic cards). Two sexual parents can theoretically produce an infinite number of kids, with not even two of them being identical! Sex, therefore, has immensely increased the genetic variation and, hence, the speed and vigor of evolution. The pre-sexual Earth’s ecosystems were likely made of only hundreds of single-celled species, whereas the current number of known species is at 10 million.
The increased variety of species has created more elaborately connected and robust ecosystems, which have been able to support the evolution of evermore complex and intelligent lifeforms. Under the forces of natural-selection, organisms must constantly adapt and evolve, not merely to gain reproductive advantage, but also to survive while pitted against opposing organisms in an ever-changing environment. Sexual reproduction has been a great advantage, and enabled continual evolution and adaptation in response to co-evolution among species.
Equipped with the vast genetic variations of the sexual revolution, plants and animals appeared on Earth and quickly colonized not just the waters, but also all the land masses that were previously uninhabited. From 600 to 66 million years ago, Earth was populated with a great variety of plants and animals, most notably the grand dinosaurs.
The truly miraculous robustness and adaptability of sexual reproduction was again demonstrated after the last environmental Holocaust of 66 million years ago, when 75% of plant and animal species went extinct in a short period of time.
66 million years ago, a huge asteroid (15 kilometers across) hit a location in today’s Mexico, creating such a violent explosion that was a billion times stronger than the atom bombs dropped on Japan; or 20 thousand times stronger than the total global nuclear arsenal in existence today! That impact and its after-effects (global incineration of plants, high surface temperature, dense ash clouds and prolonged darkness) destroyed some 99.9% of Earth’s biomass.
Ironically, the devastation caused by that asteroid’s Holocaust provided evolutionary opportunities for the most sexual and most intelligent of all the animals, the mammals. In the wake of asteroid impact, almost all the dinosaur species died, but most of the mammal species survived. The ability of mammals to survive that apocalyptic impact depended on well-organized underground living, caring for babies after birth, flexible diet and food storage, suited to surviving on a scorched Earth with minimum and variable food availability.
After the asteroid impact’s dust and ash clouds settled, sunshine returned and plant life reappeared on Earth. Then, the original mammals who were small (rat size) underwent remarkable adaptive jumps and prolific divergence into new forms and species that ultimately produced diverse animals, from horses to whales and from bats to primates.
Picture: Evolutionary tree of life.
Reference: A Natural History of the First Four Billion Years of Life on Earth, by Richard Fortey.