How to extract the first element in nested lists

I have a list, which consists of several nested lists, and I want to extract first elements in each of this nested list and put as one single list. how to do this?

   {{{b->4.72971}},{{b->4.76615}},{{b->1.91842},{b->4.80339}},{{b->1.93338},{b->4.84177}},{{b->1.94861},{b->4.88068}},{{b->1.96412},{b->4.92126}},{{b->1.97992},{b->4.96216}},{{b->1.99601}},{{b->2.01241}},{{b->2.02913}},{{b->2.04617}},{{b->2.06355}},{{b->2.08127}},{{b->2.09935}},{{b->2.1178}},{{b->2.13663}},{{b->2.15585}},{{b->2.17548}},{{b->2.19553}},{{b->2.21601}},{{b->2.23694}},{{b->2.25834}},{{b->2.28021}},{{b->2.30259}},{{b->2.32548}},{{b->2.34891}},{{b->2.37289}},{{b->2.39745}},{{b->2.4226}},{{b->2.44837}},{{b->2.47479}},{{b->2.50188}},{{b->2.52967}},{{b->2.55817}},{{b->2.58743}},{{b->2.61748}},{{b->2.64833}},{{b->2.68004}},{{b->2.71264}},{{b->2.74615}},{{b->2.78063}},{{b->2.8161}},{{b->2.85263}},{{b->2.89024}},{{b->2.92899}},{{b->2.96893}},{{b->3.0101}},{{b->3.05257}},{{b->3.09638}},{{b->3.14159}},{{b->3.18826}},{{b->3.23645}},{{b->3.28622}},{{b->3.33762}},{{b->3.39071}},{{b->3.44554}},{{b->3.50216}},{{b->3.5606}},{{b->3.62089}},{{b->3.68304}},{{b->3.74702}},{{b->3.81279}},{{b->3.88026}},{{b->3.94925}},{{b->4.01955}},{{b->4.09082}},{{b->4.16259}},{{b->4.23424}},{{b->4.30495}},{{b->4.3737}},{{b->4.43922}},{{b->4.50009}},{{b->4.55476}},{{b->4.6017}},{{b->4.63965}},{{b->4.66773}},{{b->4.68562}},{{b->4.69355}},{{b->4.69221}},{{b->4.68259}},{{b->4.66581}},{{b->4.64302}},{{b->4.61527}},{{b->4.58351}},{{b->4.54857}},{{b->4.51112}},{{b->4.47174}},{{b->4.43093}},{{b->4.38908}},{{b->4.34653}},{{b->4.30356}},{{b->4.2604}},{{b->4.21725}},{{b->4.17428}},{{b->4.13163}},{{b->4.08941}},{{b->4.04772}},{{b->4.00664}},{{b->3.96625}},{{b->3.9266}}}