<s>
Natural	B-Application
Selection	I-Application
is	O
a	O
modification	O
for	O
the	O
video	O
game	O
Half-Life	B-Application
.	O
</s>
<s>
Natural	B-Application
Selection	I-Application
v1	O
was	O
first	O
publicly	O
released	O
on	O
Halloween	B-Application
2002	O
,	O
and	O
is	O
now	O
at	O
version	O
3.2	O
.	O
</s>
<s>
Natural	B-Application
Selection	I-Application
2	I-Application
was	O
released	O
in	O
late	O
2012	O
.	O
</s>
<s>
The	O
game	O
features	O
two	O
teams	O
:	O
Kharaa	B-Application
(	O
alien	O
species	O
)	O
and	O
Frontiersmen	O
(	O
human	O
space	B-Application
marines	I-Application
)	O
.	O
</s>
<s>
The	O
visible	O
Kharaa	B-Application
"	O
units	O
"	O
are	O
actually	O
simply	O
the	O
spawn	O
of	O
the	O
real	O
Kharaa	B-Application
(	O
aliens	O
)	O
which	O
are	O
microscopic	O
life-forms	O
according	O
to	O
the	O
storyline	O
.	O
</s>
<s>
The	O
game	O
was	O
,	O
in	O
2008	O
,	O
one	O
of	O
the	O
ten	O
most	O
played	O
Half-Life	B-Application
modifications	O
in	O
terms	O
of	O
players	O
,	O
according	O
to	O
GameSpy	O
.	O
</s>
<s>
On	O
January	O
22	O
,	O
2014	O
Unknown	O
Worlds	O
released	O
the	O
source	O
code	O
for	O
download	O
on	O
a	O
GitHub	B-Application
repository	O
under	O
GPLv3	O
.	O
</s>
<s>
Natural	B-Application
Selection	I-Application
is	O
a	O
hybrid	O
FPS/RTS	O
that	O
pits	O
Marines	O
against	O
Aliens	O
(	O
"	O
the	O
Kharaa	B-Application
"	O
)	O
,	O
played	O
primarily	O
in	O
the	O
first-person	O
perspective	O
,	O
within	O
the	O
confines	O
of	O
large	O
atmospheric	O
maps	O
of	O
spaceships	O
or	O
space	O
stations	O
,	O
which	O
have	O
been	O
invaded	O
by	O
the	O
Kharaa	B-Application
.	O
</s>
<s>
There	O
are	O
two	O
game	O
modes	O
in	O
Natural	B-Application
Selection	I-Application
:	O
Classic	O
and	O
Combat	O
.	O
</s>
<s>
Classic	O
is	O
the	O
original	O
Natural	B-Application
Selection	I-Application
game	O
,	O
mixing	O
FPS	O
and	O
RTS	O
elements	O
.	O
</s>
<s>
Natural	B-Application
Selection	I-Application
was	O
one	O
of	O
the	O
first	O
multiplayer	O
,	O
first-person	O
shooters	O
to	O
incorporate	O
a	O
"	O
Commander	O
"	O
,	O
whose	O
view	O
of	O
the	O
battlefield	O
was	O
essentially	O
2-D	O
,	O
from	O
a	O
strictly	O
top-down	O
perspective	O
.	O
</s>
<s>
Kharaa	B-Application
"	O
Hive	O
Sight	O
"	O
reveals	O
teammates	O
through	O
the	O
walls	O
and	O
structures	O
of	O
the	O
map	O
,	O
this	O
was	O
originally	O
their	O
answer	O
to	O
the	O
commander	O
with	O
his	O
top-down	O
overview	O
of	O
the	O
game	O
.	O
</s>
<s>
For	O
the	O
Kharaa	B-Application
(	O
alien	O
)	O
side	O
,	O
winning	O
requires	O
destroying	O
all	O
marine	O
"	O
Infantry	O
Portals	O
"	O
,	O
ensuring	O
that	O
they	O
do	O
not	O
respawn	O
,	O
and	O
then	O
eliminating	O
the	O
rest	O
of	O
the	O
marines	O
.	O
</s>
<s>
With	O
the	O
time	O
limit	O
,	O
Marines	O
are	O
usually	O
on	O
the	O
attacking	O
side	O
and	O
Kharaa	B-Application
are	O
on	O
the	O
defending	O
side	O
.	O
</s>
<s>
GameSpy	O
hailed	O
Natural	B-Application
Selection	I-Application
as	O
"	O
possibly	O
the	O
most	O
ambitious	O
user-made	O
modification	O
ever	O
brought	O
to	O
fruition	O
.	O
</s>
<s>
This	O
multiplayer-only	O
mod	O
somehow	O
manages	O
to	O
transform	O
the	O
aging	O
Half-Life	B-Application
into	O
a	O
seemingly	O
brand-new	O
,	O
atmospheric	O
,	O
and	O
surprisingly	O
deep	O
action-strategy	O
game	O
,	O
pitting	O
hive-minded	O
aliens	O
against	O
resourceful	O
space	B-Application
marines	I-Application
,	O
"	O
and	O
"	O
Natural	B-Application
Selection	I-Application
's	O
innovations	O
are	O
too	O
numerous	O
to	O
list	O
,	O
but	O
the	O
marine	O
's	O
"	O
commander	O
"	O
mode	O
deserves	O
special	O
mention	O
.	O
</s>
<s>
Natural	B-Application
Selection	I-Application
2	I-Application
is	O
the	O
sequel	O
to	O
Natural	B-Application
Selection	I-Application
.	O
</s>
<s>
Natural	B-Application
Selection	I-Application
2	I-Application
started	O
development	O
using	O
the	O
Source	B-Application
Engine	I-Application
,	O
but	O
their	O
development	O
blog	O
later	O
announced	O
that	O
Unknown	O
Worlds	O
had	O
moved	O
to	O
the	O
Spark	O
engine	O
,	O
an	O
engine	O
they	O
created	O
themselves	O
.	O
</s>
<s>
Natural	B-Application
Selection	I-Application
2	I-Application
has	O
the	O
same	O
basic	O
gameplay	O
as	O
Natural	B-Application
Selection	I-Application
,	O
but	O
with	O
new	O
weapons	O
and	O
abilities	O
.	O
</s>
<s>
It	O
was	O
released	O
commercially	O
on	O
Steam	B-Application
.	O
</s>
<s>
In	O
addition	O
,	O
a	O
major	O
new	O
feature	O
of	O
Natural	B-Application
Selection	I-Application
2	I-Application
is	O
dynamic	O
infestation	O
,	O
released	O
in	O
December	O
2006	O
.	O
</s>
<s>
The	O
alpha	O
testing	O
of	O
Natural	B-Application
Selection	I-Application
2	I-Application
began	O
on	O
26	O
July	O
2010	O
.	O
</s>
