diff --git a/tsinfer/inference.py b/tsinfer/inference.py
index d053053c..3ba67b94 100644
--- a/tsinfer/inference.py
+++ b/tsinfer/inference.py
@@ -1644,6 +1644,19 @@ def group_by_linesweep(self):
         epoch_end = np.hstack([breaks + 1, [self.num_ancestors]])
         time_slices = np.vstack([epoch_start, epoch_end]).T
         epoch_sizes = time_slices[:, 1] - time_slices[:, 0]
+        # Find the epoch where the sum of ancestors has reached 1M as a cutoff
+        if np.sum(epoch_sizes) <= 1e6:
+            over_1M_epoch = len(time_slices)
+            over_1M_epoch_first_ancestor = self.num_ancestors
+        else:
+            over_1M_epoch = np.where(np.cumsum(epoch_sizes) > 1e6)[0][0]
+            over_1M_epoch_first_ancestor = time_slices[over_1M_epoch, 0]
+            logger.info(
+                f"1M ancestors reached at {over_1M_epoch} epoch and ancestor "
+                f"{over_1M_epoch_first_ancestor}"
+            )
+
+            # Find the first epoch with more than a 500 times the median epoch size
         median_size = np.median(epoch_sizes)
         cutoff = 500 * median_size
         # Zero out the first half so that an initial large epoch doesn't
@@ -1653,13 +1666,17 @@ def group_by_linesweep(self):
         # the median epoch size. For a large set of human genomes the median epoch
         # size is around 10, so we'll stop grouping by linesweep at 5000.
         if np.max(epoch_sizes) <= cutoff:
-            large_epoch = len(time_slices)
-            large_epoch_first_ancestor = self.num_ancestors
+            large_epoch = over_1M_epoch
+            large_epoch_first_ancestor = over_1M_epoch_first_ancestor
+            logger.info("No large epochs found, using count cutoff")
         else:
             large_epoch = np.where(epoch_sizes > cutoff)[0][0]
             large_epoch_first_ancestor = time_slices[large_epoch, 0]
+            logger.info(
+                f"Large epoch found at {large_epoch} with {epoch_sizes[large_epoch]} "
+                f"ancestors and ancestor {large_epoch_first_ancestor}"
+            )
         logger.info(f"{len(time_slices)} epochs with {median_size} median size.")
-        logger.info(f"First large (>{cutoff}) epoch is {large_epoch}")
         logger.info(f"Grouping {large_epoch_first_ancestor} ancestors by linesweep")
         ancestor_grouping = ancestors.group_ancestors_by_linesweep(
             start[:large_epoch_first_ancestor],
@@ -1672,6 +1689,11 @@ def group_by_linesweep(self):
             ancestor_grouping[next_epoch] = np.arange(*time_slices[epoch])
             next_epoch += 1
 
+        # Assert that every ancestor appears once in ancestor grouping
+        assert (
+            len(set(np.hstack(list(ancestor_grouping.values())))) == self.num_ancestors
+        )
+
         # Remove the "virtual root" ancestor
         try:
             assert 0 in ancestor_grouping[0]