Decay of radioactive isotopes, differentiation of earths iron core, gravity-driven compression, and meteoric bombardment all caused early earth to

Decay of radioactive isotopes, differentiation of earths iron core, gravity-driven compression, and meteoric bombardment all caused early earth to be much hotter than at present. 
The Earth was formed roughly 4.5 billion years ago. the atmosphere was predominantly consisted of methane (CH4) and ammonia (NH3). These greenhouse gases and their radioative forcing kept the Earth’s atmosphere warm.