  Over 60 years after the discovery of the first quasar, more than $275$ such
sources are identified in the epoch of reionization at $z>6$. JWST is now
exploring higher redshifts ($z\gtrsim 8$) and lower mass ($ \lesssim 10^7
\Msun$) ranges. The discovery of progressively farther quasars is instrumental
to constraining the properties of the first population of black holes (BHs), or
BH seeds, formed at $z \sim 20-30$. For the first time, we use Bayesian
analysis of the most comprehensive catalog of quasars at $z>6$ to constrain the
distribution of BH seeds. We show that the mass distribution of BH seeds can be
effectively described by combining a power law and a lognormal function
tailored to the mass ranges associated with light and heavy seeds, assuming
Eddington-limited growth and early seeding time. Our analysis reveals a
power-law slope of $-0.70^{+0.46}_{-0.46}$ and a lognormal mean of
$4.44^{+0.30}_{-0.30}$. The inferred values of the Eddington ratio, the duty
cycle, and the mean radiative efficiency are $0.82^{+0.10}_{-0.10}$,
$0.66^{+0.23}_{-0.23}$, and $0.06^{+0.02}_{-0.02}$, respectively. Models that
solely incorporate a power law or a lognormal distribution within the specific
mass range corresponding to light and heavy seeds are statistically strongly
disfavored, unlike models not restricted to this specific range. Our results
suggest that including both components is necessary to comprehensively account
for the masses of high-redshift quasars, and that both light and heavy seeds
formed in the early Universe and grew to form the population of quasars we
observe.
