Mars has never been "like earth". It was however "more like earth".
It had an atmosphere, it had liquid water on the surface, it had seasons and a climate. So far we do not know if it had life or not, it had conditions that may have allowed life but we have so far found no evidence of anything living there.
However it has lower gravity than earth and no magnetic field to protect it from the solar wind. (It did have a magnetic field at some point but that has gone now and was probably always weaker than Earth's, it's also possible according to some theories that it only covered the southern half of the planet!). This means that the atmosphere was gradually lost into space, as the atmospheric pressure dropped the water either froze or evaporated and was also lost until in the end you have the Mars we see today.