Context . Clusters are common sites of star formation , whose members display varying degrees of mass segregation .
The cause may be primordial or dynamical , or a combination both .
If mass segregation were to be observed in a very young protostellar cluster , then the primordial case can be assumed more likely for that region .

Aims . We investigated the masses and spatial distributions of pre-stellar and protostellar candidates in the young , low-mass star forming region Serpens South , where active star formation is known to occur along a predominant filamentary structure .
Previous observations used to study these distributions have been limited by two important observational factors : ( 1 ) sensitivity limits that leave the lowest-mass sources undetected , or ( 2 ) resolution limits that can not distinguish binaries and/or cluster members in close proximity .

Methods . Recent millimeter-wavelength interferometry observations can now uncover faint and/or compact sources in order to study a more complete population of protostars , especially in nearby ( D < 500 pc ) clusters .
Here we present ALMA observations of 1 mm ( Band 6 ) continuum in a 3 \times 2 arcminutes region at the center of Serpens South .
Our angular resolution of \sim 1 \mbox { $ { } ^ { \prime \prime } $ } is equivalent to \sim 400 au , corresponding to scales of envelopes and/or disks of protostellar sources .

Results . We detect 52 sources with 1 mm continuum , and we measure masses of 0.002 - 0.9 solar masses corresponding to gas and dust in the disk and/or envelope of the protostellar system .
For the deeply embedded ( youngest ) sources with no IR counterparts , we find evidence of mass segregation and clustering according to : the Minimum Spanning Tree method , distribution of projected separations between unique sources , and concentration of higher-mass sources near to the dense gas at the cluster center .

Conclusions . The mass segregation of the mm sources is likely primordial rather than dynamical given the young age of this cluster , compared with segregation time .
This is the first case to show this for mm sources in a low-mass protostellar cluster environment .