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Articles with such headlines regularly appear in the media and are a clear exaggeration used to attract the attention of readers, while in the very text of such articles there is usually nothing about that Mars was exactly the blooming planet.
About 4.5 billion years ago, when the planet of the solar system was only formed, the Earth and Mars were similar. Both planets were bunches of molten substance. Due to the fact that Mars is significantly less than the Earth, as well as due to the lack of large and greater remoteness from the Sun Mars cooled significantly faster than the Earth.
Therefore, on Mars earlier than on Earth, a solid surface appeared and, possibly, oceans of liquid water. Further cooling led to the loss of Mars of its magnetic field, without which the solar wind for billions of years simply blocked almost the entire atmosphere from Mars. This led to the evaporation of the oceans, and the fact that it was not evaporated — frozen.
At the moment there are different time estimates when this happened, the most realistic say that Mars lost a magnetic field after 400-600 million years after the formation, and atmosphere in 700 million — 1 billion years. Taking into account the fact that part of the time Mars was in the molten state, then the hypothetical Martian life had no 1 billion years to develop.
It would be difficult to say that these 300-400 million years would be enough to ensure that any complex forms of life appear on Mars. In front of our eyes there is only one example of the development of life — our land. If you compare how life developed on Earth developed, it is extremely doubtful that some organisms could appear in such a short time on Mars.
So oceans on Earth have formed about 4.41 billion years ago. The first undeniable signs of the existence of life are dating rates 3.45 billion years ago. At this stage, life was the simplest bacteria. It is them in the petrified form find archaeologists.
There are hypotheses and an earlier life on the basis of finds of petrified structures that may have biotic origins. The first such finds refer to 4.28 billion years ago, i.e. 100-150 million years after the formation of the oceans. However, the biotic origin of such finds is disputed by many scientists.
As for a more difficult life, the first signs of the emergence of multicellular organisms in the form of microscopic algae appear approximately 2.1 billion years ago, and the evolution of animals begins only a billion years ago.
If we assume that on Mars, the development of life went similar way, then for the first 300-400 million years after the formation of the oceans and before the loss of Mars, a significant part of the atmosphere could be formed except for the simplest unicellient organisms.
Life on Mars, if such and was just not enough time to develop to complex organisms, and even more so capture the surface of the planet and create an ecosystem on it at least completely comparable to the earth. Therefore, Mars could not be a green planet, although life on him could exist in the past and may exist now.