With the experimental evidences of $\overline B{}^0\to p\bar p$ and $B^−\to\Lambda\bar p$ decays, it is now possible to extract both tree and penguin amplitudes of the charmless two-body baryonic $B$ decays for the first time. The extracted penguin-tree ratio agrees with the expectation. Using the topological amplitude approach with the experimental results on $\overline B{}^0\to p\bar p$ and $B^−\to\Lambda\bar p$ decay rates as input, predictions on all other $\overline B_q\to{\cal B}\overline{\cal B}$, ${\cal B}\overline{\cal D}$, ${\cal D}\overline{\cal B}$ and ${\cal D}\overline{\cal D}$ decay rates, where ${\cal B}$ and ${\cal D}$ are the low lying octet and decuplet baryons, respectively, are given. It is non-trivial that the results do not violate any existing experimental upper limit. From the analysis it is understandable that why $\overline B{}^0\to p\bar p$ and $B^−\to\Lambda\bar p$ modes are the first two modes with experimental evidences. Relations on rates are verified using the numerical results. We note that the predicted $B^−\to p\overline{\Delta^{++}}$ rate is close to the experimental bound, which has not been updated in the last ten years. Direct CP asymmetries of all $\overline B_q\to{\cal B}\overline{\cal B}$, ${\cal B}\overline{\cal D}$, ${\cal D}\overline{\cal B}$ and ${\cal D}\overline{\cal D}$ modes are explored. Relations on CP asymmetries are examined using the numerical results. The direct CP asymmetry of $\overline B{}^0\to p\bar p$ decay can be as large as $\pm50\%$.