At present, the evidence suggests that life may have existed in the past on Mars, or not. However, the scientific consensus is that we assume life never arose on Mars, and will continue to do so until evidence pops out and bites us in the mass spectrometer.
There is no evidence of life on Mars right now.
NASA just discovered what looks to human palaeontological eyes just like trace fossils, of organisms burrowing through or living in mud, a long time ago. Further study is under way. Meanwhile, this is what the rock of interest looks like:
The trace fossil looking things would be the semi-squiggly darkish tubes shown here in this somewhat blurry blow-up:
Those could be mineral deposits that formed in mud in the absence of any living organism. NASA’s info on this find is here.
On a planet with life, these look like some evidence of that life. On a planet that never had life, these look like natural mineral concretions. But if only life could be so simple!
In my own personal experience, these look closest to iron or manganese (or both) deposits that form in bogs. However, that does not mean that they look to me like natural concretions, because I’m pretty sure that those bog deposits occur with the help of bacteria. In other words, trying to figure out what these Martian items are by reference to “natural mineral concretions” on Earth is going to be more problematic than people may be thinking, because such mineral concretions are often signs of life on our planet even if they are not, themselves, specifically, direct fossils or traces of living organisms.
But here is the thing that I find interesting: The chances that this photograph represents life on Mars is either very high or very low in the eyes of a given observer depending on the answer to this question: How likely to you think life is to begin with? If you think life is very likely to emerge where there are things like sunlight or a chemical source of life-usable energy, and liquid water, is very very high, then we can be pretty certain that Mars at one time had life, and we are simply waiting for NASA to find it. It would be a surprise to NOT find it eventually, and since evidence absent is hard evidence to parse without resorting to useless aphorisms, we’ll just have to wait a long time before giving up on the idea. Alternatively, if you think that life is very unlikely to get started given the basic raw materials, so that what we have on Earth is special and rare, then you should be very agnostic about anything that comes along in the way of lifey-looking stuff from the angry red planet.
But here is the other thing. Which way a particular individual, especially a scientist and especially a NASA scientist, is actually going to break on this has less to do with gut feeling or understanding the origin of life or any of that, than it has to do with worrying that people are going to think you are nuts.
As a person who fully expects that life is likely (generally speaking), and therefore, that it probably emerged on Mars at some point, in some form, here is what I think is going to happen:
Scientists will remain publicly agnostic about life ever existing on Mars. They will become annoyed at anyone that suggests otherwise, but they will always leave the possibility open. They will give a little more wiggle room for a singe celled bacteria like form of life, and bridle at any suggestion of multi-celled life (if these are “trace fossils” that’s multi-celled life, if it was Earth). They will also get mad at anyone who discuses life on Mars mainly in terms of multi-celled life, as thought such an interlocutor was dissing the poor Martian bacteria. Which we shall remain agnostic about.
Then we’ll find definitive evidence of life on Mars. It may be bacteria-like, or it may include colonial or basic multi-cellular forms. Or something else. Whatever.
Then, since the two planets we a) know about and b) can reach, have evidence of life, we’ll thereafter assume life is easy, as it were, and widespread in the Universe, and eventually, only historians will remember when we thought differently!
I just hope that those squiggles aren’t dirt that fell out of the Mars rover’s treads as it went by this site at an earlier time.