No Space Aliens? Are We Alone in the Universe?

Why Are There No Visitors From Outer Space?

Part I – Planet and sun type; simple cell biology

Originally, scientists thought that intelligent life should be somewhat common in the universe. All one had to do was to estimate the number of planets in the habitable zones of their stars in our galaxy, and there were an unlimited number of candidates. The prebiotic molecules that help kick-start life can be found throughout the universe. So life made up of single cells is probably common on many planets.

However, a closer look at how life formed on Earth suggests that a number of very unlikely events had to occur for intelligent life to develop from basic single-celled organisms. First, the planet and its sun have to be favorable.

Our Sun, a very stable and bright star called a G-type main-sequence star, formed approximately 4.6 billion years ago. It will remain fairly stable for billions of years more. It is estimated to be brighter than about 85% of the stars in our Milky Way galaxy, most of which are red dwarfs. This stability, along with its brightness, has given life on Earth a generous amount of time to evolve, using the abundant solar radiation as an energy source. This itself is not that unusual, but it puts Earth in a more advantageous position than many other planets.
The Earth, a standard rocky planet, was hit by the Mars-sized planet Theia early in its formation, resulting in a partial merger giving Earth a metallic core larger than normal, a tilted axis, and a moon much larger in relation to its size than the moon of any other planet in our solar system. The larger core gave it a stronger, longer-lasting magnetic field that shields Earth from solar winds that can harm living organisms. The slightly tilted axis gave it seasons. The large moon, tidally locked, gave it rotational stability along with tides which ebb and flow, making it easier for living organisms to develop.
The Earth is within the habitable zone of the sun, where it is neither too hot for liquid water to exist, nor too cold so that water freezes. This habitable zone will continue for one billion years more.
Earth has a lot of water. Much of this was originally bound up in the larger molecules that formed Earth. But much of it also came from meteors and comets that entered the atmosphere. The water is enough to cover three-fourths of the Earth's surface and is a great environment for life to develop and evolve.

The Earth with its sun is far more advantageous for the development of intelligent life than most other planets in other solar systems.

The first primitive single-celled organisms to appear were prokaryotes, classified into bacteria and archaea. Prokaryotes have a problem. As they grow larger, their volume and energy needs increase much faster than their surface area. Since they obtain energy by interacting with their environment through their outer surface, they very quickly reach a size limit and must stay tiny.

A chance solution to the energy problem resulted when a new cell type, the eukaryote, emerged. Unlike prokaryotes, a eukaryote has an internal membrane-bound nucleus and other internal organelles, one being a mitochondria, which provides far more energy than could be produced by the cell itself.

Bottleneck 1 – No Eukaryotes

Only eukaryotes have the energy to evolve into complex multicellular organisms. The manner in which eukaryotes came about on Earth, and the limited time frame in which they could have developed, suggests that their presence was an unlikely chance event. Without eukaryotes, intelligent life cannot evolve.

Eukaryotes apparently came about when [1] the production of oxygen by photosynthetic bacteria led to a world-wide catastrophe which forced other prokaryotes to mutate to be compatible with oxygen, or die. [2] Eukaryotes developed as a merger, called a symbiogenesis, between two cell types, an anaerobic (oxygen incompatible) archaeon and an aerobic (oxygen compatible) bacterium. The aerobic bacterium became the mitochondria, and the merger became an alternative way for at least one species of archaea to survive oxygenation.

There was a relatively short time frame when oxygen stress was beginning to kill many prokaryotes, but before all anaerobic cells died. The merger came about in part because of a unique characteristics of the two cells.

The archaeon had developed a DNA mutation that provided greater flexibility of its cell membrane, allowing it to surround and ingest objects touching it. The aerobic bacteria could perform a chemical exchange with the archaeon that reduced its oxygen load, and supplied it with energy, in the form of ATP molecules as a waste product.

This symbiotic arrangement meant that the archaeon did not consume the aerobic as it normally would, but kept it as an organelle. The short time frame for this arrangement to occur, plus the rare DNA capability and compatibility of both cells, make the formation of eukaryotes a chance event less likely to happen on other planets.

Eukaryotes, with their better energy source, are able to grow much larger than prokaryotes, create more elaborate DNA, and evolve into complex multicellular organisms that lead to intelligent life.

So on many planets, eukaryotes probably do not form, and intelligent life never emerges. This may be why we have no visitors from other planets. While the probability of eukaryotes forming on most planets seems unlikely, it happened on Earth, resulting in the evolution of modern humans.

In Part II we look at other roadblocks to the development of intelligent life on a planet, assuming that eukaryotes do develop.