Generally I think people tend to underestimate how quickly life arose on Earth but underestimate how quickly it took for life to turn into complex life. Just judging from how things went on Earth we should expect cemosynthetic life to be relatively common but photosynthesis to rarely arise, much less eukaryotic cells or multi-cellular animals. I drew out a timeline comparing how long it took various stages of life to arise on Earth versus how long it took Mars to lose all its surface water. Even on Earth we humans didn't arise until relatively close to when the expanding Sun will render the surface uninhabitable.
> Surface water sufficient to sustain microorganisms was found on Mars by Viking, Pathfinder, Phoenix and Curiosity;
Ultraviolet (UV) activation of the Martian surface material did not, as initially proposed, cause the LR reaction: a sample taken from under a UV-shielding rock was as LR-active as surface samples;
Complex organics, have been reported on Mars by Curiosity’s scientists, possibly including kerogen, which could be of biological origin;
Phoenix and Curiosity found evidence that the ancient Martian environment may have been habitable.
The excess of carbon-13 over carbon-12 in the Martian atmosphere is indicative of biological activity, which prefers ingesting the latter;
The Martian atmosphere is in disequilibrium: its CO2 should long ago have been converted to CO by the sun’s UV light; thus the CO2 is being regenerated, possibly by microorganisms as on Earth;
Terrestrial microorganisms have survived in outer space outside the ISS;
Ejecta containing viable microbes have likely been arriving on Mars from Earth;
Methane has been measured in the Martian atmosphere;
microbial methanogens could be the source;
The rapid disappearance of methane from the Martian atmosphere requires a sink, possibly supplied by methanotrophs that could co-exist with methanogens on the Martian surface;
Ghost-like moving lights, resembling will-O’-the-wisps on Earth that are formed by spontaneous ignition of methane, have been video-recorded on the Martian surface;
Formaldehyde and ammonia, each possibly indicative of biology, are claimed to be in the Martian atmosphere;
An independent complexity analysis of the positive LR signal identified it as biological;
Six-channel spectral analyses by Viking’s imaging system found terrestrial lichen and green patches on Mars rocks to have the identical color, saturation, hue and intensity;
A wormlike feature was in an image taken by Curiosity;
Large structures resembling terrestrial stromatolites (formed by microorganisms) were found by Curiosity;
a statistical analysis of their complex features showed less than a 0.04 percent probability that the similarity was caused by chance alone;
No factor inimical to life has been found on Mars.
That is the evidence. Science is often done indirectly. We know that the Sun is made up of hydrogen gas but no one has been there. We know black holes exist, and that the big bang took place 15 billion years ago, that atoms are made up of protons, electrons and neutrons, and that gravitational waves ripple through spacetime. No one has directly seen these things, but we know them to be true based on indirect evidence.
Similarly, the above list of facts is indirect evidence for life on Mars.
The author is not claiming that he knows for sure. Based on his involvement in the project, he has strong reasons to believe it. He is calling for more research in this area. It's similar to NASA's claim that life on Mars is likely to be found by 2021.
It is "inconclusive" at best.