What form of life was in the Precambrian era.
The Cambrian period was when most of the body plans of todays complex multicellular animals appeared: Segmented and unsegmented worms, arthropods (trilobites were extremely abundant in the Cambrian), starfish, molluscs and the first chordates (early jawless fish) all made their appearance. Fossils suddenly become abundant in rocks from this period. This extraordinary proliferation happened relatively rapidly and seemingly all at once, so biologists speak of the 'Cambrian explosion'. It wasn't that long ago (relatively speaking), about 500 million years ago.
So if the earth is about 4.5 billion years old, and if it's true (I'm a little skeptical) that evidence of life extends back to at least 3.5 billion years ago, what was happening in that 3 billion year interval before the Cambrian explosion? What kind of life was found on the earth during those years?
Generally speaking, single-celled life. For most of the period, probably procaryotic life, simpler less complex types of cells like today's bacteria.
Today's plant and animals (and fungi) are composed of eucaryotic cells, which are more complex. There's a lot of controversy about when eucaryotic cells appeared, but it certainly happened during the Precambrian.
There's evidence that multicellular eucaryotic life started appearing during the late Precambrian. Fossils have been discovered illustrating what's called the 'Ediacarian biota' a whole assortment of soft-bodied things that may have been animals or may have been plants (the plant and animal lines may not have diverged yet). Sponges may be survivors of this earliest multicellular Precambrian life.
https://en.wikipedia.org/wiki/Ediacara_biota
Interestingly, few of these Ediacarian fossil lifeforms seem to be ancestral to any of the forms expressed in the Cambrian explosion (that resulted in us). So it's possible that there was a mass-extinction that wiped out and aborted most of the Ediacarian developments and cleared the decks for more successful alternatives.
Was Precambrian era in water , was there dry land ?
Probably some of both, like today. But life hadn't colonized the land yet in those days, apart from some of the ubiquitous bacteria probably, so life was largely found in the oceans.
There's evidence of tremendous environmental change in the Precambrian. During the early Precambrian, the early earth didn't have oxygen gas in its atmosphere. But the early oceans were apparently filled with photosynthetic bacteria that produced free oxygen as a byproduct of their photosynthesis. This created a tremendous crisis for life and probably a mass-extinction, since oxygen is highly reactive and was probably extremely poisonous to the life of the time. It breaks up life's biochemistry through oxidation, and likely exterminated all lifeforms that couldn't evolve ways to deal with it or weren't protected somehow in anaerobic environments. Eventually most life (with the exception of a few forms of anaerobic bacteria) evolved biochemistries that became dependent on oxygen. It's tempting to speculate that the appearance of eucaryotic cells with their (endosymbiotic?) mitochondria might have been associated somehow with life's adaptation to the so-called 'oxygen holocaust'.
Temperatures varied tremendously during these three billion years too. There were apparently periods when the entire earth was frozen solid in super ice-ages, even at the equator. One of these 'snowball-earth' episodes may be what separates the Ediacarian biota from the Cambrian.