What early life in the universe was probably like
It is possible that, at least in some circumstances, life could have been born when the universe was still relatively young.
The first galaxies appeared as early as one billion years after the Big Bang, about 12.8 billion years ago. But the mere presence of stars and galaxies was not enough for the development of life. Very few generations of stars had followed each other until then, therefore elements heavier than hydrogen and helium, necessary for life, were present only in very small quantities.
For the necessary elements to form, it is safe to assume that we have to wait at least another billion years or two. This way at least some star-forming region could have generated stars with the right metallicity, a parameter that indicates the percentage of “heavy” elements that are present, and an indispensable requirement for life as we know it.
At the time, the universe was a much more turbulent place than it is now, populated by very massive and short-lived stars, that emitted radiation harmful to life and had a rapid generational turnover. The typical planets in which life could have been born would be, like ours, in the peripheries of galaxies, far from high energy events.
From those planets you would see a different night sky, with more stars, smaller and/or less defined galaxies, and a greater variety of colors. Most of the stars, being on average more massive than today, were blue. In addition, the night sky would be patterned with red spots, indicating star-forming regions, as the stars’ birth rate was much higher.
The typical star around which those typical planets with life could have been found, would therefore be type F stars, larger and brighter than the sun, that emit a lot of blue and ultraviolet light.
It is possible that this higher energy radiation could have accelerated the rate of mutation and therefore life’s evolution, which is also preferable, given that type F stars have a shorter lifespan than the sun.
Assuming that life on typical planets was not too different from that on earth (with autotrophic and heterotrophic creatures, etc …), their vegetation could have evolved to have either a blue or a yellow color.
This is because plants have two types of mechanisms to harness their star’s light. Either they evolve to use the predominant wavelength, or they reflect it completely, exploiting the remaining frequencies.
The emission spectrum of the sun peaks in the frequency corresponding to green, the color that plants reflect, using red and blue instead. But purple-colored plants that only absorb green are very possible too, as in the case of cyanobacteria.
So on typical planets, we would have flora of the same predominant color as the typical star (blue) or its “opposite” (yellow).
I think the first option is the more likely one, both because it is what plants actually do on earth, and because absorbing all the blue and ultraviolet light would probably overheat the plants.
Even the animals may have had blue or turquoise colors, to reflect the harmful radiation of their star, and blend in with the rest of the environment. Probably almost all the organisms of those typical planets would be able to see in the ultraviolet, and have patterns on the body detectable only in that frequency to communicate with each other.
Due to the reduced metallicity of the stars, there was less material available for the formation of planets. This implies that the typical planets would have been smaller than earth, and lower gravity translates into animals that could reach really large sizes and lower atmospheric pressure.
It is possible that the thinner air of typical planets would have prompted organisms to adapt to lower oxygen levels. So they would most likely have been ectotherms and with a long lifespan.
Given the amount of ionizing radiation emitted by typical stars, it’s not unlikely that every organism on typical planets, after colonizing the land, would have evolved a way to correct mutations in their DNA, like the extremophile Deinococcus Radiodurans has really done here on earth.