\documentclass[beamer]{standalone}
\usepackage{tikz}
\usetikzlibrary{arrows,calc, patterns, positioning, decorations.pathreplacing,decorations.markings,decorations.pathmorphing,backgrounds}
\usetikzlibrary{shapes,snakes}
\usetikzlibrary{external}
\tikzexternalize % activate! 
\begin{document}
\begin{standaloneframe}

% If overlays do not work, use \only<1-n>{...} where n is the max overlay
% \only<1-1000>{
  \begin{tikzpicture}[]
    \coordinate (d0) at (-1.5,0);
    \coordinate (c0) at (-1.5, 1);
    \coordinate (h0) at (-1, 2);
    \draw[fill=black] (c0) circle (0.08) node[right] {$\tau$};
    \draw[fill=black] (h0) circle (0.08) node[left] {$\sigma$};

    \coordinate (d1) at (2,0);
    \coordinate (c1) at (2,1);
    \coordinate (h10) at (1.5, 2);
    \draw[fill=black] (h10) circle (0.08) node[left] {$\sigma$};
    \draw[dashed] (h10) -- ++(1,0);
    \coordinate (h11) at (2.7, 2.5);
    \draw[fill=black] (h11) circle (0.08) node[right] {$\tau$};

    \coordinate (d2) at (5.5,0);
    \coordinate (c2) at (5.5,1);
    \coordinate (h20) at (5, 2);
    \draw[dashed] (h20) -- ++(1,0);
    \coordinate (h20p) at (5.25, 2.5);
    \draw[fill=black] (h20p) circle (0.08) node[left] {$\sigma$};
    \coordinate (h21) at (6, 2);
    \draw[fill=black] (h21) circle (0.08) node[right] {$\tau$};

    % \coordinate (d2) at (5.5,0);
    % \coordinate (c2) at (5.5, 1);
    % \coordinate (h2) at (6, 2);
    % \draw[fill=black] (c2) circle (0.08) node[right] {$\sigma$};
    % \draw[fill=black] (h2) circle (0.08) node[right] {$\tau$};

    \draw (d0) to (c0) -- (h0);
    \draw (d1) -- (c1) -- (h10);
    \draw (c1) -- (h11);

    \draw (d2) to (c2);
    \draw (h20p) -- (h20) -- (c2) -- (h21);

    % \draw (0.5,0) -- (0.5,1) -- (0,2);
	% ...   
  \end{tikzpicture}
% }
\end{standaloneframe}
\end{document}